Preheat oven to 200'C, gas mark 6. Place the squash, onion in oil in a roasting tin, season and toss. Bake for 10 minutes. Stir in the mushrooms, cook for 15 minutes. Stir in the sage and chilli, cook until tender (about 5 minutes). Mix in the spinach and set aside.
Meanwhile, lightly oil a 23cm (9in) round tart tin and lightly beat the eggs and yolk. Brush a filo sheet with oil and line the tin, with overhang. Repeat with the remaining filo, overlapping to cover the tin. Brush with oil, scrunch the edges and prick the base. Bake for 7 minutes. Flatten the centre, brush with egg and cover the edges with foil, and bake for 7 minutes, until golden.
Reduce the heat to 180'C, gas mark 4. Mix the remaining egg with the creme fraiche and cheese. Season and stir into the cooked vegetables. Spoon into the case and bake for 40-45 minutes, until set.