Preheat oven to 400°F (200°C) and generously spray a 6-cup muffin tin with cooking spray.
Halve 3 of the turkey slices crosswise. Line each of the 6 muffin cups with 1½ slices of turkey so the cups are fully covered and form a cup shape with no holes.
Place about 1 tablespoon shredded cheddar and ½ tablespoon diced green onion into the center of each turkey-lined muffin cup.
In a large bowl, whisk together 2 whole eggs, 6 egg whites, and 1/4 cup Buffalo sauce until combined. Season with a pinch of salt and pepper.
Carefully divide the egg mixture evenly among the 6 muffin cups, filling almost to the top of each cup.
Bake for 15–18 minutes, until the eggs are set and slightly puffed. Remove from oven and let cool for a minute.
Drizzle additional Buffalo sauce over the muffins if desired, then serve immediately.
