Heat a skillet over medium-low heat, add cooking spray or olive oil. Sauté the diced onions for 2-3 minutes until they become translucent. Add spinach and cook for about 2 minutes until wilted. Let cool.

In a large mixing bowl, whisk together the eggs and egg whites for about 30 seconds. Mix in the milk, kosher salt, and black pepper. Stir in the shredded cheddar cheese, sliced cherry tomatoes, and cooled vegetable mixture until evenly distributed.

Preheat your oven to 375°F. Pour the egg mixture into a well-greased baking sheet, spreading it evenly. Bake for 25-30 minutes, or until the eggs are set and firm. Allow to cool slightly before cutting into squares.

While the eggs bake, slice the English muffins in half and place them in the oven. Toast for about 10 minutes, adding the cheese slices to the bottom halves for the last 3 minutes to melt.

Carefully cut the baked egg mixture into squares. Place a piece on the toasted English muffin base, sandwich it with the top half.