Prep your ingredients: Slice the mushrooms, dice the onion, and mince the garlic. Rinse and pat the spinach dry.
Heat the pan: Set a nonstick skillet over medium heat.
Add the oil and let it warm until it shimmers.
Sauté aromatics: Add the onion with a pinch of salt. Cook 2–3 minutes, stirring, until it softens. Stir in the garlic and cook 30 seconds, just until fragrant.
Cook the mushrooms: Add the sliced mushrooms.
Spread them out so they make contact with the pan. Cook 3–4 minutes, stirring occasionally, until they release their moisture and turn golden around the edges. Season with a little salt and pepper.
Wilt the spinach: Add the spinach to the pan.
Toss with the mushrooms until it wilts down, about 1 minute. If using red pepper flakes, sprinkle them in now.
Add the egg whites: Pour in the egg whites and reduce the heat to medium-low. Let them sit for 10–15 seconds, then gently push with a spatula from the edges toward the center, creating soft curds.
Scramble to your preferred texture: Continue gently folding and pushing the egg whites until just set and slightly glossy.
This usually takes 2–3 minutes. Avoid overcooking.
Finish and season: Turn off the heat. Taste and adjust salt and pepper.
If using cheese or herbs, fold them in now. A small squeeze of lemon brightens the flavors.
Serve: Plate immediately while warm. Add extra herbs or a grind of pepper on top if you like.