Rinse and slice tomato into 4 slices. Rinse spinach. If using avocado, peel, remove pit, and slice now.
Split each muffin in half. Set muffin tops aside.
In a medium skillet over medium heat, heat oil.
Gently break eggs into skillet. Be careful not to break the yolk. Cook about 1 minute. Carefully flip each egg over with a rubber spatula. Cook until yolk is slightly firm, about 1–2 minutes. Top each muffin bottom with cooked egg.
Add spinach to skillet with a splash of water and pinch of salt. Cover and cook until wilted, about 3 minutes.
Top the egg with tomato and spinach. If using avocado, add avocado slices now. Cover with the muffin tops to make a sandwich.