Destalk mushrooms and put on a grill pan or baking tray, along with tomato slices. Dot everything with a little garlic butter (about a tablespoon) and pop under a medium-hot grill for 5 minutes, until tender.
Meanwhile, heat a small knob of garlic butter in a frying pan. Crack in eggs and cook for a few minutes to set the white.
Toast burger buns. Top each bottom half with a few baby spinach leaves, a grilled mushroom, a tomato slice and a fried egg. Sandwich with the bun tops and serve with oven-baked chips.