Slice onion and mushrooms; set aside.

Heat medium nonstick skillet, lightly coated with spray, over medium-high heat.

Add onion and mushrooms; cook, stirring frequently, for 5 minutes, or until onion is soft.

Add spinach, and cook, stirring frequently, 1 minute, until wilted. Remove from pan; set aside. Reduce heat to medium-low.

Coat same pan with spray. Add egg whites; cook, stirring frequently, for 3 to 4 minutes, or until eggs are just set. Remove from heat.

Arrange egg mixture on half of tortilla; top evenly with onion mixture and cheese.

Fold tortilla in half; place cheese-side down in medium nonstick skillet over medium-high heat. Cook for 4 to 5 minutes, turning once, until cheese is melted and tortilla is golden brown.

Top with pico de gallo; serve immediately.