Form the turkey sausage into four thin patties.
Heat the olive oil in a skillet over medium heat and cook the patties for 4 to 5 minutes per side until browned and 165°F.
Transfer the patties to a plate, add the spinach and cook for 2 minutes until wilted and dry.
Pour in the egg whites, season with salt and black pepper, and fold slowly until set and 160°F.
Divide the egg whites into four portions and sprinkle each with feta.
Fill the toasted muffins with one turkey patty and one portion of spinach egg whites.