Preheat the oven to 190°C (170°C fan) and lightly butter a 20cm × 20cm baking dish. The butter film prevents the apple juices from sticking and adds a faint richness to the bottom layer. You should see a thin, glossy sheen across the base and sides of the dish.

Combine the apple wedges, 80g demerara sugar, cinnamon, nutmeg, 2 tbsp flour, and lemon juice in a large bowl. Toss gently with your hands or a spatula until every wedge is coated. The sugar will begin to draw moisture from the apples, and the mixture will look glossy and speckled with dark spice flecks. The lemon juice stops the apples browning and sharpens the overall flavour.

Tip the apple mixture into the prepared dish and spread it into an even layer. Press the wedges down gently so they sit flat. There should be no large gaps; the fruit should fill the dish from edge to edge. The surface will look rough and juicy, with a few sugar crystals still visible.

In a separate bowl, combine the jumbo oats, 100g flour, 120g demerara sugar, and salt. Stir with a fork to break up any lumps. The dry mixture should look uniform, with oats and flour evenly distributed. Add the flaked almonds now if using.

Pour the melted butter over the dry oat mixture and stir with a fork until every oat and flour particle is coated. The mixture will clump together in places; this is exactly what you want. It should smell deeply buttery and look like damp, golden rubble with no dry flour pockets remaining. If it seems too dry, add another tablespoon of melted butter.

Scatter the oat topping over the apples in an even, craggy layer. Do not press it down. Leave peaks and gaps so the steam can escape and the topping can brown in the oven. The fruit should be completely covered, but the surface should look rough and uneven.

Place the dish on a baking tray to catch any overflow, then bake for 45–50 minutes. After 30 minutes, the kitchen will smell of cooked apple and cinnamon. At 45 minutes, the topping should be deep golden brown, and the apple juices should be bubbling visibly around the edges of the dish. If the topping is browning too quickly, tent loosely with foil for the final 10 minutes.

Remove from the oven and let the dish rest on a wire rack for at least 15 minutes. During this time, the juices will thicken slightly, and the topping will firm up. The dish will still be warm but no longer volcanic. A rested crisp slices cleanly; a rushed one collapses into a soupy mess.