Line a 34 x 20cm tin with non-stick baking paper and heat the oven to gas mark 5, 190°C, fan 170°C.
Combine the cornmeal or polenta, baking powder, bicarbonate of soda and salt in a mixing bowl. Add the egg, yogurt, milk, 100g of the grated cheese, remaining olive oil, chopped sage and two-thirds of the squash. Mix to just combine, then spoon into the lined tin, smoothing the top.
Scatter with the reserved sage leaves, squash and cheese. Bake for 35 mins or until risen, firm and golden. Cool in the tin for 10 mins then turn out and cut into squares.
If freezing, when ready to eat, bake at the same temperature for an extra 10 – 15 mins.