1 Preheat oven to 180°C. Lightly spray with oil a 19 x 34cm fluted tart tin with removable base.
2 Steam, boil or microwave sweet potato for 10–12 minutes, adding asparagus for last minute. Drain. Refresh asparagus under cold running water; drain. Set aside.
3 Meanwhile, spray a medium non-stick frying pan with olive oil and set over medium heat. Sauté onion for 5 minutes. Add garlic and zucchini; cook, stirring, for 2–3 minutes, or until wilted.
4 Place one filo sheet on a clean work surface. Spray sheet with oil. Continue layering with remaining sheets. Lift the pastry into the tin. Tuck in the long edges.
5 Whisk the eggs, buttermilk and the parmesan together in a large bowl. Combine the sweet potato and zucchini mixtures and spread over filo pastry base. Scatter over roasted capsicum and top with asparagus. Carefully pour the egg mixture over the tart. Scatter with extra parmesan.
6 Bake for 40–45 minutes. Serve sprinkled with basil leaves.