01 -  Heat the olive oil in a nonstick skillet over medium heat. 
 02 -  Add the red onion and sauté for 2 minutes until softened. 
 03 -  Add the baby spinach and cherry tomatoes, cooking for 1 to 2 minutes until the spinach is wilted and tomatoes begin to soften. 
 04 -  Pour in the egg whites, season with salt and black pepper, and gently scramble until just set, approximately 2 to 3 minutes. 
 05 -  Warm the whole wheat tortillas in a dry skillet or microwave for 20 seconds until pliable. 
 06 -  Divide the egg white mixture evenly between the tortillas. 
 07 -  Top each tortilla with avocado slices, cheddar cheese (if using), salsa, and fresh cilantro. 
 08 -  Roll up each tortilla tightly into a burrito. Serve immediately or wrap in foil for on-the-go.