In a bowl, beat the whites with salt and black pepper. If using fresh spinach - wash, dry and slice it. If you take frozen - drain excess water. Cut the tomato into small cubes.
Grease six muffins with vegetable oil. Spread the spinach and the slices of tomato evenly. Fill with protein and send to the oven, heated to 180 ° C, for 15 minutes.
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