Melt butter in a large frying pan on medium heat. Cook the mushrooms and garlic for 5-7 minutes, flipping halfway through, until mushrooms are tender. Season with a little salt and pepper.
Cut Freya’s German Sourdough Buns in half and toast in a frying pan on high heat (or leave them fresh). Spread the bottom halves of the buns with aioli, then top with spinach, mushrooms, capsicum, mozzarella slices, another mushroom (if using two per burger), more baby spinach and a generous dollop of pesto. Sandwich together with the top halves of the buns and serve immediately.