Breakfast casserole

Name: Breakfast casserole
Ingredients: 250g chestnut mushrooms, sliced; 1½ tbsp vegetable oil, plus extra for the tin (optional); 6 veggie sausages, sliced into bite-sized pieces; 1 large red onion, finely sliced; 2 red peppers, deseeded and sliced; 8 eggs; 250ml whole milk; 125g cheddar, grated; 10g chives, finely sliced