Preheat the oven to 375°F.
Heat a medium-sized skillet over medium heat. Then, add ground beef and diced onion. Cook, breaking up the ground beef into small pieces until browned. When browned, season with salt and freshly ground pepper. Stir to combine. Turn off the heat.
In a small bowl, whisk together eggs, heavy cream, cumin, salt, and freshly ground pepper. Set aside.
Spray an 8-inch square baking pan with avocado oil. Then, transfer chopped roasted poblano peppers to the pan. Spread evenly.
Add the browned beef mixture to the baking pan. Spread in an even layer over the chopped roasted poblano peppers. Then, top with shredded Monterey jack cheese and shredded cheddar cheese. Pour the beaten egg mixture over the keto Mexican casserole. Bake in the preheated oven for 15-20 minutes or until cheeses melt and the beaten egg mixture sets. Remove keto Mexican casserole from the oven. Cut and serve with sour cream, guacamole, or salsa. Sprinkle with chopped parsley, green onions, or cilantro. Enjoy!