Heat the oven to 175 C / 350 F with a rack in the middle. Line a 23 cm / 9-inch square baking pan with parchment, leaving an overhang on two sides.

In a medium bowl, whisk the flour, oats, brown sugar, 1 tsp thyme, 1 tsp lemon zest, baking powder and salt. Stir in the melted butter until the mixture looks like damp, clumpy sand and holds together when squeezed. Set aside about 180 g, roughly one third, for the topping.

Press the remaining mixture firmly and evenly into the pan, corners included, using the bottom of a measuring cup. Bake for 15 minutes, until the edges look pale golden and the surface is set.

While the base bakes, toss the blackberries in a second bowl with the granulated sugar, cornstarch, lemon juice, 1 tsp lemon zest and 1 tsp thyme. Crush about a third of the berries with a fork until the mixture is juicy but still chunky.

Spread the filling over the hot base, right to the edges. Squeeze the reserved mixture into clumps and scatter them over the top, leaving small gaps. Bake for 28 minutes, until the topping is golden and the juices bubble thickly at the edges and center. Frozen berries need 3 to 5 minutes more.

Cool the bars in the pan on a wire rack for 60 minutes, then refrigerate for 30 minutes. Lift the slab out by the parchment and cut it 4 by 4 into 16 squares, wiping the knife between cuts. Cutting while warm makes the filling run and the topping slide.