Heat a skillet over medium-low heat. Coat it with cooking spray or add the olive oil.

Add the diced onion and cook for 2–3 minutes, until softened and translucent.

Add the fresh spinach and cook for approximately 2 minutes, tossing occasionally, until completely wilted. Remove the vegetables from the heat and allow them to cool slightly.

Preheat the oven to 375°F (190°C). Thoroughly coat a 9x13-inch baking pan with cooking spray to prevent the eggs from sticking.

Crack the whole eggs into a medium mixing bowl and add the egg whites. Whisk for about 30 seconds until combined.

Add the milk, kosher salt, and black pepper and whisk again.

Stir in the shredded cheddar, sliced cherry tomatoes, and cooled spinach and onion mixture.

Pour the egg mixture evenly into the prepared baking pan. Bake for 25–30 minutes, or until the eggs are completely set in the center. Remove from the oven and allow to cool for several minutes.

While the eggs bake, split the English muffins and arrange them cut-side up on a separate baking sheet. Toast in the oven for approximately 10 minutes.

After about 7 minutes, place one cheddar slice over each bottom muffin half. Return to the oven for the final 3 minutes, or until the cheese melts.

Use a round cookie cutter, jar lid, or appropriately sized glass to cut six portions from the baked egg mixture. Alternatively, cut the eggs into squares with a knife.

Assemble each Healthy Breakfast Sandwich with one portion of baked egg and one cheese-topped English muffin.

Serve warm or allow the sandwiches to cool before storing them for meal prep