In a large non-stick pan, heat the olive oil over medium heat.
Cook the bell pepper and red onion until soft, around 4-5 minutes.
While cooking, whisk together the eggs, milk, salt and pepper.
Add the egg/milk mixture to the peppers/onions, stirring frequently, until eggs are nearly cooked through.
Add the spinach and feta and fold into the eggs, stirring until spinach is wilted and eggs are cooked through.
Remove eggs from heat.
Spray a separate large non-stick pan with spray oil, and heat over medium. Add the tortillas, then on half of each tortilla, spread about ½ cup of the egg mixture. Top with around ⅓ cup of shredded mozzarella cheese. Fold the second half of the tortilla over, then cook for 2 minutes, or until golden brown.
Flip and cook for another minute or so (until golden brown).