Preheat the oven to 200°C, gas mark 6.
Combine the eggs, milk, Cheddar cheese, garlic paste and season generously.
Grease a baking dish with the butter and arrange the croissant chunks, mushrooms, leeks, onion and cherry tomatoes. Distribute the garlic paste on top.
Pour over the egg mixture.
Place in the oven and bake until golden brown.