There are few things more quietly satisfying than pulling a golden, cheese-scented traybake from the oven on a grey afternoon. This one takes the familiar partnership of sweet, slow-caramelised onions and sharp mature cheddar and bakes it into a sturdy, sliceable slab that holds together in a lunchbox, survives a rucksack and tastes just as good cold as it does warm from the tin.

It is the sort of baking that suits British kitchens and British weather: hearty, unfussy and endlessly adaptable. Whether you are feeding a crowd after a muddy walk, packing a picnic for a blustery beach or simply want something in the fridge that isn't a biscuit, these slices earn their place.

A traybake built for salt air and appetite

Savoury traybakes sit somewhere between a scone, a savoury cake and a thick omelette, and that is precisely their charm. They need no proving, no rolling and no particular skill, just a bowl, a wooden spoon and a lined tin. Because everything is baked flat, you get plenty of crisp, chewy edge and a generous golden top, which is where the best flavour lives.

They also travel beautifully. A slab of this, wrapped in greaseproof paper and cut into thick fingers, is the ideal companion to a flask of tea at the end of a coastal path, or a bowl of tomato soup on a wet Sunday. The texture is firm enough to eat one-handed in the wind, yet soft enough to feel like a treat rather than rations.

Take your time with the onions

The single most important step in this recipe is the least dramatic one. Onions need to be cooked slowly, in a wide pan, until they collapse into sticky, amber strands. Rushing them over a high heat gives you harsh, slightly bitter pieces; patience gives you sweetness that spreads right through the bake.

• Slice two large onions thinly, around 400g prepared weight, and warm two tablespoons of rapeseed oil in a wide frying pan over a low to medium heat.
• Add a teaspoon of soft brown sugar and a good pinch of fine sea salt. Cook gently for 25 to 30 minutes, stirring every few minutes, until deep gold and jammy.
• A splash of balsamic vinegar or Worcestershire sauce in the final two minutes lifts the sweetness and scrapes up all the sticky goodness from the base of the pan.
• Spread the onions on a plate to cool completely. Warm onions will slacken the batter and give you a denser slice.

If you have a glut of onions, caramelise double the amount and keep half in the fridge for a future bake. They will happily sit there for four days.

The cheese, and a coastal flourish

Use a mature or extra-mature cheddar, and grate it coarsely so you get pockets of molten cheese rather than a uniform orange haze. A crumbly, salty coastal-style cheese works particularly well here, and a handful of finely crumbled Cheshire or Caerphilly mixed through the batter adds a lovely lactic tang.

For a seaside note without any gimmickry, stir in a teaspoon of dried seaweed flakes with the herbs, or scatter a few flakes over the top with the final grating of cheese. Seaweed brings a savoury, mineral depth that makes the onions taste sweeter. Fresh thyme and chives are the classic partners, but a little chopped wild garlic in spring or finely snipped lovage in summer does wonderful things. Finish with a scattering of poppy or nigella seeds, or a spoonful of porridge oats for a craggy, golden crust.

The recipe

Makes 12 generous slices. You will need a 20cm by 30cm tin, lined with baking parchment, and an oven preheated to 190°C fan, 200°C conventional, gas mark 6.

• 250g plain flour
• 2 teaspoons baking powder
• 1 teaspoon mustard powder
• 1 teaspoon fine sea salt
• 100g cold butter, cubed
• 200g mature cheddar, coarsely grated
• 2 large eggs
• 150ml whole milk
• 2 tablespoons natural yoghurt
• The cooled caramelised onions
• 1 teaspoon fresh thyme leaves, plus 2 tablespoons chopped chives
• A further 50g grated cheddar and 1 teaspoon seeds or oats, for the top

• Rub the cold butter into the flour, baking powder, mustard powder and salt until the mixture looks like coarse breadcrumbs. Stir through the 200g of cheddar, the herbs and the cooled onions.
• Beat the eggs with the milk and yoghurt, then pour into the dry ingredients. Mix with a knife or spatula until just combined. Stop while the mixture still looks a little lumpy; overworking it makes the slices tough.
• Spread into the lined tin, levelling the surface with the back of a spoon. Scatter over the remaining cheddar and the seeds or oats.
• Bake for 30 to 35 minutes, until risen, deeply golden and firm to a gentle press in the centre. A skewer should come out clean.
• Leave to cool in the tin for 15 minutes before lifting out onto a board. Cut into twelve slices while still slightly warm.

Seasonal swaps through the year

Spring calls for wild garlic, young leeks and a little grated goat's cheese in place of some of the cheddar. In summer, fold in sweetcorn or roasted peppers and serve the slices with a sharp tomato chutney for a picnic. Autumn is the time for diced apple, sage and a stronger, saltier cheese, and winter rewards a leek and Stilton version, eaten warm alongside soup or a bowl of beans. A handful of chopped smoked bacon or flaked smoked mackerel turns it into a full meal, though both need to be cooked and cooled first.

Slicing, storing and picnicking

Keep the slices in an airtight tin for two days, or in the fridge for up to four. They freeze excellently, wrapped individually, for up to three months; thaw overnight and refresh in a moderate oven for ten minutes. Cut them into fingers for lunchboxes, thicker slabs for a picnic, and small squares for a drinks table. However you serve them, warm or cold, they are best eaten outdoors, with salty air and a decent view.