Preheat oven to 350ºF. Coat 6 jumbo muffin cups with cooking spray. Brush both sides of trimmed bread slices with melted butter. Press 1 bread slice into each muffin cup, molding it to the shape of the cup. The bread corners will rise out of the edges of the cup. Bake until lightly toasted, about 15 minutes.
Break 1 egg into each toast-lined cup and season with salt and pepper. Add chopped tomatoes and onion, if using, dividing equally among muffin cups. Top with cheese and bake until cheese melts and eggs are set, 15 to 20 minutes. Scatter on parsley, if using.
Tip: If you prefer, you can scramble the eggs then pour them into the toast cups.