Cut the avocado in half, remove the pit, and scoop out half (about 1/2 avocado). In a small bowl, mash with a fork and stir in 1/2 tsp lime or lemon juice and a pinch of salt and pepper. Set aside.

Toast English muffin

Split the English muffin and toast until lightly golden. Keep warm.

Cook egg whites

Lightly grease a nonstick skillet with 1 tsp olive oil (or use cooking spray) and warm over medium. In a small bowl, whisk the 4 tbsp egg whites (and the optional whole egg, if using) with 1/4 tsp salt and 1/8 tsp black pepper. Pour into skillet and cook gently, stirring once, until mostly set (about 1–2 minutes). For a folded omelet-style, allow to set without stirring, then fold to fit the muffin (about 1–2 minutes). Remove from heat.

Assemble sandwich

Spread the mashed avocado evenly on one toasted English muffin half. Top with baby spinach (about 1/2 cup), then place the cooked egg whites (or folded egg) on top. Add tomato slices (2 slices) and the optional cheese slice. Cap with the other toasted muffin half.

Optional Spread / Seasoning

If using a spread, mix 1 tsp plain Greek yogurt (or light mayonnaise) with 1/4 tsp hot sauce (optional), a pinch of salt and pepper. Spread a thin layer on the inside of the top or bottom muffin half before assembling for extra flavor.

Choice of Side

Choose one side: serve the sandwich with 3/4 cup mixed berries, or one small sliced apple, or 3/4 cup baby carrot sticks.

Finishing

Serve warm. To make ahead: keep avocado mashed separately with a little extra lemon juice to prevent browning, store egg and muffin separately and assemble just before eating.