Heat the olive oil in a skillet and cook the bell pepper for 5 minutes until soft.
Add the spinach and cook for 3 minutes until wilted and the skillet is dry.
Spread the vegetables in a greased 9-by-13-inch pan, pour over the egg whites and season with salt and black pepper.
Bake at 350°F for 14 to 16 minutes until the egg whites reach 160°F, then cool and cut into six squares.
Fill each muffin with egg white and Pepper Jack, wrap in parchment and foil, and freeze for up to 2 months.
Remove the foil and microwave in parchment until the center reaches 165°F, usually 2 to 3 minutes.