Spray a medium nonstick skillet with olive oil spray. Set the pan over medium heat, adding the broccoli, onion, and garlic. Cook, stirring occasionally, for 4 to 6 minutes, or until the onion is almost tender.
Add in the spinach, cooking for 1 to 2 minutes, or until wilted. Remove the vegetables and set them on a plate, covering them to keep warm.
In a small bowl, whisk the egg whites and pepper until the egg whites bubble lightly. Coat the pan again with olive oil spray. Set over medium heat, and pour the egg white mixture into the pan.
Cook, lifting the edges with a spatula as they begin to set and tipping the pan for uncooked egg whites to run underneath, for 2 to 3 minutes, or until almost set. Flip the omelet.
Spoon the reserved vegetables over the egg white mixture. Sprinkle the cheese evenly over the top. Cover and cook for 1 to 2 minutes, or until the cheese is melted. Fold the omelet in half.
Move to a serving plate, seasoning the omelet with salt and top with picante sauce, if desired. Serve immediately.