Preheat the oven to 350°F (175°C).
Cook the Ground Beef:
Heat a nonstick skillet over medium-high heat. Add the ground beef and taco seasoning. Cook until browned on all sides.
Add minced onion and diced green chilies to the skillet. Cook until the onion is tender and slightly browned, about 5 minutes.
Prepare the Sour Cream Mixture:
In a bowl, combine the sour cream, mayonnaise, and half of each of the shredded cheddar cheese and shredded pepper jack cheese. Mix well and set aside.
Make the Bisquick Dough:
In another bowl, combine the Bisquick and water. Stir until a soft dough forms.
Assemble the Casserole:
Spread the Bisquick dough into a greased 9-by-13-inch casserole dish.
Top the Bisquick dough with the cooked ground beef mixture.
Spread the sour cream mixture over the beef layer.
Add Cheese Topping:
Sprinkle the remaining shredded cheddar cheese and shredded pepper jack cheese on top.
Bake the Casserole:
Bake in the preheated oven for 30 minutes, or until bubbly and browned on top.
Once done, remove from the oven and let it cool slightly before serving.
