While muffins are baking, spray a 9x13" baking pan (I used Pyrex) with non-stick cooking spray.
Heat a skillet over medium-high heat over the stove. Crumble the sausage links into the skillet and saute until browned and cooked through, breaking apart with a wooden spoon as needed. Arrange in a single layer in bottom of baking dish.
Add olive oil to skillet. Once hot, add garlic and sliced peppers. Saute until fragrant, tossing to coat evenly in oil. Once tender, add on top of sausage in baking dish and pour egg whites over sausage and peppers.
Add thawed spinach onto a double-layered paper towel. Squeeze excess moisture out of spinach over sink.
Whisk whole eggs with onion powder, salt, and pepper. Add spinach and stir to break apart, then pour into baking dish and gently stir to incorporate into egg whites.
Bake at 350 degrees for 25-30 minutes or until no longer jiggly in the middle. Cut into 7 slices.