In a medium skillet over medium heat, add a drizzle of cooking spray or olive oil. Sauté the diced onion for about 3-4 minutes until translucent, then add the fresh spinach. Stir continuously until the spinach wilts, which takes about 2-3 minutes. Once done, remove the veggies from the heat and let them cool while you prepare the egg mixture.

In a large mixing bowl, whisk together the 6 large eggs, 4 egg whites, ¼ cup milk, ½ teaspoon kosher salt, and ¼ teaspoon ground black pepper until well combined. Fold in the cooled spinach and onion mixture, along with the ¼ cup shredded cheddar cheese and 1 cup sliced cherry tomatoes. Make sure everything is evenly distributed to create a flavorful Healthy Breakfast Sandwich filling.

Preheat your oven to 375°F (190°C). Lightly coat a baking dish with cooking spray to prevent sticking. Pour the prepared egg mixture into the dish, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until the eggs are firm and lightly golden on top. You can check for doneness by inserting a toothpick; it should come out clean.

Slice the 6 English muffins in half and place them on a baking sheet. When there are about 5 minutes left for the eggs, pop the baking sheet in the oven to toast the muffins. For the last 3 minutes, add a slice of cheddar cheese on top of each muffin half so that they melt perfectly while you finish baking the eggs.

Once the egg mixture has cooled for a few minutes, slice it into rounds or squares that fit neatly on your muffin halves. Start by placing the egg pieces on the toasted muffin bottoms, and then top them with the other muffin half, creating a satisfying Healthy Breakfast Sandwich. Make sure to press down gently for a perfect bite!