Blue Mushroom Omelet

Ingredients : 4 eggs 1 tablespoon whole milk 4 pinches ground black pepper 2 pinches garlic salt or to taste 2 teaspoons olive oil 4 cremini mushrooms sliced 2 tablespoons chopped red onion 1 cup baby spinach coarsely chopped ½ ounce crumbled Stilton cheese ½ cup shredded part-skim mozzarella cheese...