Preheat the oven to 180°C fan (350°F).
Place the walnuts and oats in a food processor and blitz until you get a scruffy mealy texture. Add the baking powder, salt, fennel seeds and flour to the food processor and pulse to combine. Add the olive oil and 2 tbsp of the non-dairy milk and pulse together until you get a cohesive dough. You may need to add more milk to get it to come together.
Crumble the dough into the base of a 23-cm (9-inch) springform cake tin (or a loose-based tart tin) and use your fingertips to press it down into an even layer over the base and up the sides of the tin (in a springform tin it should come up about 3cm high). Reserve a little bit of the dough for patching up cracks later on. Prick the crust all over with a fork.
Place the cake tin/tart tin on a baking sheet and then into the oven. Bake for 10-15 minutes until it looks dry. Remove from the oven and use the reserved pastry to patch up any cracks that may have formed. Set the crust aside.