Toast bread to desired level.
Cook egg whites over med-low heat in a small skillet. Cover with a lid to help the egg whites cook evenly. The egg whites should be cooked in about 2-3 minutes. Remove from heat and set aside.
Spread the mashed avocado on both slices of bread. Top each slice of bread with half of the egg whites and the pepper jack cheese.
Season with salt and pepper, to taste. If desired, top with a drizzle of sriracha sauce. Serve.