Set the oven to preheat at 350°F and lightly grease a non-stick or silicone muffin pan.
Pat the cherry tomatoes dry using a paper towel to remove extra moisture.
Warm a non-stick skillet over medium-high heat, then add bell peppers and onions; cook for 2 to 3 minutes until softened.
Add the crumbled sausage, spinach, and tomatoes to the pan and cook everything together for another 2 minutes.
Spoon a generous tablespoon of the cooked mixture into each muffin cup.
In a separate bowl, whisk the egg whites along with salt and pepper until blended.
Pour the egg whites gently over the fillings in each muffin cup, making sure to leave about a quarter inch of space at the top.
Bake the muffins in the oven for 23 to 30 minutes until they have puffed and a toothpick inserted comes out clean.
Carefully run a knife around each muffin to loosen, then remove and serve warm.
Store leftovers in a sealed container in the fridge and consume within 5 days.