Preheat the oven to 180°C, fan 160°C, gas mark 4.
Put the butternut squash and sweet potato in a bowl with a little water and over loosely with clingfilm. Cook in the microwave for 5 minutes on high. Drain and tip into a large bowl.
Add the eggs, garlic, chillies, feta, hard cheese and crème fraîche. Season with freshly ground black pepper and mix gently until everything is combined.
Heat the oil in a 20cm ovenproof frying pan and pour in the fritatta mixture. Top with the sage leaves and cook over a medium-low heat for 10 minutes.
Transfer to the oven and cook for a further 20 minutes. Remove from the oven, slice into wedges and serve with the sweet herb salad and slices of ciabatta.
Cook's tip: Make a quick dressing for the herb salad by whisking together 3 tablespoons of olive oil, 1 tablespoon of vinegar and 1 teaspoon of Dijon mustard.