Berry Almond Crumble Traybake

STEP 1: LINE THE TIN

Butter the tin and line it with parchment, leaving a generous overhang on both long sides. This is a three-layer bake with wet fruit in the middle: it will not turn out of a tin and it cannot be cut cleanly inside one, so the overhang is how the whole slab comes out in one piece. Heat the oven to 350F / 180C (160C fan) with a shelf in the middle.

STEP 2: MAKE THE CRUMBLE FIRST AND CHILL IT

Rub the cold butter into the flour and fine salt with your fingertips until it looks like coarse breadcrumbs with some pea-sized pieces left in. Stir in the light brown sugar and the sliced almonds. Make this before the sponge: the crumble needs to be cold when it goes on, and making it first buys it a rest in the fridge while everything else happens.

STEP 3: ADD THE WATER AFTER THE RUB-IN, NOT DURING

Sprinkle 2 teaspoons of ice water over the surface of the mixture and toss it through with a fork or with your fingertips spread wide. Do not rub, squeeze or knead at this stage. Add the third teaspoon only if the mixture is still entirely loose. Take handfuls and press them lightly into clumps, break those clumps up roughly with your fingers, spread the lot on a baking sheet and chill until needed.

STEP 4: PREPARE THE FRUIT

Put the blueberries, raspberries and blackberries in a bowl, add the cornstarch and the tablespoon of caster sugar, and turn them over gently with your hands until every berry carries a dull matte film. The cornstarch binds the juice the fruit will throw in the oven and is the difference between a fruit layer and a wet seam. Use fresh fruit: frozen berries release several times as much liquid and will flood the sponge. Do this last, immediately before assembly, because fruit left sitting in sugar begins to weep.

STEP 5: MAKE THE ALMOND SPONGE

Beat the butter and caster sugar together for a full 4 to 5 minutes, until noticeably paler and grown in volume. Add the eggs one at a time, beating well between each; if it looks about to curdle, add a spoonful of the flour. Beat in the almond extract and vanilla extract. Fold in the flour, baking powder, ground almonds and salt by hand until just combined, then fold in the milk. You want a batter that drops reluctantly off the spoon: stiff enough to hold fruit up, slack enough to spread level.

STEP 6: ASSEMBLE IN THE RIGHT ORDER

Spread the sponge batter into the tin and level it properly with an offset spatula or the back of a spoon; whatever unevenness you leave here shows up in the cut pieces later. Scatter the coated fruit evenly over the batter and press it in very lightly, just enough to seat it. Do not stir it through. Take the crumble straight from the fridge and scatter it over the fruit in an even layer, right to the edges, with a little relief left in it: flattening the crumble is what turns it into a sheet.

STEP 7: BAKE

Bake for 50 to 60 minutes. At around 35 minutes the crumble should be taking color at the edges; if it is going dark before the middle is set, lay a sheet of foil loosely over the top and carry on. It is done when the crumble is golden all over, the fruit is bubbling at the edges of the tin, and a skewer pushed into a corner away from a berry comes out clean. Do not pull it early: a traybake this deep carries a lot of residual moisture, and an under-baked center will not firm up on cooling.

STEP 8: COOL COMPLETELY, THEN LIFT

Leave it in the tin on a wire rack for at least 90 minutes. Warm sponge under warm fruit is structurally hopeless and will tear the moment a knife goes in. When it is fully cold, lift the whole slab out by the parchment overhang onto a board.

STEP 9: GLAZE AND FINISH

Warm the sieved apricot jam with the teaspoon of water until it is just fluid, and brush a thin film over the fresh berries with a pastry brush. Set them over the crumble in whatever pattern you want the cut pieces to carry: if you are cutting into 24, place them in a 4 x 6 grid so each piece gets its own. Scatter the toasted sliced almonds. The glaze is what makes the fresh fruit read as deliberate rather than dropped on, and it stops it drying out.

STEP 10: CUT IDENTICAL PIECES

Measure, do not eyeball. Mark the long side into six and the short side into four with the tip of a knife, then cut with a long thin blade in single downward strokes. No sawing, which drags the crumble into the fruit. Wipe the blade between cuts. Identical pieces were an explicit requirement of the brief and are half of what a traybake is judged on.