Sauté the Aromatics: Heat a skillet over medium-low heat with olive oil or cooking spray. Add chopped onions and sauté for 2–3 minutes until translucent.

Wilt the Greens: Add the baby spinach to the skillet and toss frequently for about 2 minutes until wilted. Remove from heat and allow the mixture to cool completely.

Whisk the Base: In a medium mixing bowl, whisk the eggs and egg whites for 30 seconds. Incorporate the milk, salt, and pepper, whisking until well combined.

Fold in Fillings: Gently stir the shredded cheddar, halved cherry tomatoes, and the cooled onion-spinach mixture into the egg base.

Bake the Egg Slab: Preheat your oven to 375°F (190°C). Generously grease a 9×13-inch baking pan or rimmed baking sheet. Pour the egg mixture in an even layer and bake for 25–30 minutes until the center is firm.

Toast the Muffins: While the eggs bake, arrange sliced English muffins on a separate tray. Place them in the oven for 10 minutes total.

Melt the Cheese: After the muffins have toasted for 7 minutes, place a cheese slice on the bottom halves and return to the oven for the remaining 3 minutes to melt.

Cool the Eggs: Remove the eggs from the oven and let them sit for at least 5 minutes. This ensures they hold their shape when cut.

Portion the Eggs: Use a round cookie cutter, mason jar lid, or a knife to cut the egg slab into 6 portions that match the size of your muffins. Use a spatula to carefully lift them.

Assemble: Place one egg portion onto the toasted muffin half and top with the cheesy half. Serve immediately or wrap for future meals.