Preheat the oven to 180C/350F/Gas 4.
Beat the eggs and milk together in a bowl. Add the cheese and bread and mix until well combined. Season with salt and freshly ground black pepper.
Heat the oil in a frying pan and fry the onion until soft. Add the bacon and cook until golden-brown and crisp. Stir in the butternut squash, mozzarella and sage
Spoon half the eggy bread mixture onto the base of a 25cm/10in ovenproof dish. Add half of the squash mixture.
Repeat the layering process in point 4. Finish by sprinkling over with parmesan and bake in the oven for 30 minutes, or until the mixture has set and the cheese is bubbling on top.
Serve the bacon strata with green vegetables and garlic bread.