Heat oil in a skillet over medium-low heat until hot. Add diced onions and cook until translucent. Add the spinach and cook until wilted. Remove from heat.
Preheat your oven to 375 degrees. Spray a 9x13'' pan with cooking spray and set aside.
In a mixing bowl, whisk together the eggs and egg whites until smooth. Add the milk, salt, and pepper, and whisk to combine.
Add the cherry tomatoes, onions, and spinach to the egg mixture, stirring well to combine.
Pour the egg mixture into the prepared baking pan. Bake for 25-30 minutes, until the eggs are fully cooked, and the center is set. Let the eggs cool and rest for 25 minutes.
While the eggs are baking, cook the Canadian bacon or regular bacon according to package instructions.
Toast the English muffins to your liking while the eggs are cooling.
Use a circular cookie cutter or a large glass to cut circles out of the baked eggs, creating egg patties.
Assemble the sandwiches: on the bottom half of an English muffin, layer cheese, an egg patty, and Canadian bacon or regular bacon. Top with the other half of the English muffin.
To store, wrap each sandwich in aluminum foil. Refrigerate for up to 4-5 days. For freezer storage, thaw a sandwich in the fridge overnight. Keep it wrapped in foil and heat in the oven at 450°F for 10-15 minutes, or microwave for 3-5 minutes if in a rush.
Serve and enjoy your Millie's Good Morning Sandwich!