Heat oven to 350°F (175°C). Line an 8 inch square metal pan with parchment, leaving two lifting handles. Stir flour, sugar and salt together.

Stir melted butter and vanilla into the dry ingredients until damp clumps form. Reserve 3/4 cup of this mixture in a separate bowl. Press the remaining mixture firmly and evenly over the lined pan base.

Bake the base for 15–18 minutes, until set with very pale golden edges. Meanwhile combine the reserved mixture with almonds and oats, rubbing lightly to make irregular crumbs.

Carefully spread jam over the warm base, leaving about 1/4 inch clear at the edges. Scatter crumble evenly over the jam without packing it into a solid lid.

Return to the oven for 25–30 minutes, until the topping is golden and jam bubbles near the edges. Shield loosely with foil if almonds brown too fast.

Cool in the pan on a rack for about 2 hours. Lift by parchment onto a board and cut into 12 rectangles. Hot jam can burn and warm bars crumble easily, so do not rush cutting.