Heat the oil in a large, deep frying pan with an ovenproof handle over a medium-low heat, add the onion and butternut squash, then cover loosely and cook gently, stirring frequently, for 18–20 minutes or until softened and golden.
Lightly beat the eggs, thyme, sage and ricotta in a jug, then season well with salt and pepper and pour over the butternut squash.
Cook for a further 2–3 minutes until the egg is almost set, stirring occasionally with a heat-resistant rubber spatula to prevent the base from burning.
Slide the pan under a preheated grill, keeping the handle away from the heat, and grill for 3–4 minutes or until the egg is set and the frittata is golden. Slice into 6 wedges and serve hot.