Preheat the oven to 190°C (Gas mark 5).
Unroll the ready-rolled croissant pastry onto a baking tray, press the seams together to form a single crust, then bake for 8–10 minutes until lightly golden.
In a medium bowl, whisk together the eggs, milk, salt and ground black pepper until frothy.
Melt the butter in a non-stick frying pan over a medium heat. Pour in the egg mixture and scramble until just set but still fluffy.
If using the optional vegetables, add the diced green pepper and red onion to a separate non-stick frying pan over a medium heat. Stir in the cooked, crumbled sausage and fry for 3–5 minutes until the vegetables are just softened.
Remove the pastry from the oven and let it cool briefly. Spread the scrambled eggs evenly over the crust, then top with the sausage and vegetable mixture (if using) and sprinkle over the shredded cheddar cheese.
Return to the oven for 3–5 minutes, or until the cheese has melted. Remove from the oven, slice and serve hot.