Combine eggs, egg whites, and Greek yogurt in a bowl; beat by hand until thick and fluffy.
Spray cooking spray inside of canning jars. Layer sausage, bell pepper, and spinach into the 2 jars. Pour egg mixture evenly into each jar; fill to 3/4 full to allow room to rise. Place jars onto a small baking sheet.
Bake in the preheated oven until eggs are firm, 25 to 30 minutes. Remove from oven; cool for 5 to 10 minutes. Season with garlic powder, salt, and pepper.