Preheat the oven to 350ºF and lightly coat a 9X13” baking dish with cooking spray. Set aside.
Heat a heavy-bottomed skillet over medium heat, then add in the ground beef. Crumble and cook until no pink remains. Drain.
Sprinkle the taco seasoning onto the drained ground beef, along with ⅔ cup of water. Stir well to combine, and cook just until a thickened sauce forms.
Transfer the ground beef to the prepared baking dish, spreading it into an even layer.
Sprinkle the diced green chilies on top of the ground beef layer, aiming to spread it out as evenly as possible.
Combine the shredded Monterey Jack and cheddar cheese and place them on top of the ground beef and green chilies. Set aside.
Separate the egg whites from the yolks, placing the egg whites in a medium-sized bowl, and the yolks into a separate medium-sized bowl.
Add the heavy cream (or evaporated milk), flour, salt, and cayenne pepper to the yolks and whisk well to combine.
Use an electric hand mixer to whip the egg whites until stiff peaks form.
Transfer the whipped egg whites to the yolk mixture and fold it in gently until fully combined.
Pour the egg mixture on top of the cheese into an even layer, spreading it out using the back of a spoon.
Place the tomato slices on top of the egg layer and place the dish in the oven.
Bake for 40-50 minutes, or until the egg layer is set and the cheese layer is fully melted and bubbly.
Allow it to cool for 5-10 minutes before slicing and serving.
Notes
Without ground beef: The original recipe does not call for ground beef or taco seasoning. I added this to give it more of a traditional casserole feel, but you can absolutely leave it out! Just leave out the ground beef, taco seasoning, and water. Instead, increase the shredded cheddar cheese to a full 16 ounces. Shredded cheese: For the best results I recommend using cheese from the block and shredding it by hand. This tastes the best and it melts better than pre-shredded cheese.*Low carb, keto, and gluten-free: The recipe as written is very low carb! Just swap out the 1 tablespoon of flour that is used as a thickener for almond flour or gluten-free flour blend - both have been tested with great results!