Toast the muffin. Split the whole-wheat English muffin and toast it until golden. A crisp base helps the sandwich hold up and adds texture.
Mix the eggs. In a small bowl, whisk egg whites with one whole egg.
Season with a pinch of salt and pepper.
Cook the eggs. Heat a nonstick skillet over medium. Lightly coat with cooking spray. Pour in the eggs, then gently swirl or fold as they set.
Aim for a soft, folded square to fit the muffin. Remove from heat when just cooked.
Warm the turkey. In the same pan, add the turkey for 30–60 seconds to warm and slightly brown the edges. This adds flavor without extra calories.
Build the sauce. Stir together light mayo (or Greek yogurt) with mustard.
Spread on the toasted muffin halves.
Assemble the sandwich. Layer spinach or arugula on the bottom half, then the eggs, warm turkey, and reduced-fat cheese. Top with tomato or onion if using, then cap with the other muffin half.
Optional melt. Return the assembled sandwich to the skillet over low heat, cover for 1–2 minutes to melt the cheese. This step makes it feel café-level without extra fuss.
Finish and season. Add hot sauce or a dash of everything bagel seasoning if you like a kick.
Serve hot.