Oil skillet or omelet pan with cooking spray and heat over high heat for a minute.
Add the mushrooms and scallions; cook over high heat until the mushrooms are just cooked through, stirring often (about 3 minutes).
Remove from heat and add the thyme, keep warm.
Using the same small skillet, again lightly oiled, add half of the egg substitute. Cook over medium heat, lifting the sides of the eggs to allow uncooked eggs to flow under.
Once the bottom is lightly browned, carefully flip the omelet with a spatula to brown the other side.
Spoon half of the mushroom mixture onto the omelet and fold in half. Transfer the omelet to a plate and keep warm (in a warm oven and covered in foil is a good way to do that).