Preheat the oven: Set to 350°F (175°C). Grease a 9x13-inch baking dish with a light coat of oil or nonstick spray.

Cook the beef: Warm a large skillet over medium-high heat. Add ground beef and cook, breaking it up, until browned and no longer pink. Drain excess fat if needed.

Season and add vegetables: Stir in diced onion, bell peppers, and jalapeño. Sauté 4–5 minutes until softened. Sprinkle in taco seasoning, add the drained tomatoes with chiles, and simmer 2 minutes. Taste and adjust salt and pepper.

Make the crust: In a bowl, whisk Bisquick, milk, and eggs until smooth. Pour into the prepared baking dish, spreading it evenly.

Layer the beef: Spoon the beef and pepper mixture over the crust batter. Don’t press it down—let the crust rise around it as it bakes.

Mix the creamy topping: In a small bowl, combine sour cream and mayonnaise until smooth. Spread this mixture gently over the beef layer.

Add cheese: Sprinkle shredded cheddar evenly across the top.

Bake: Place in the oven for 30–35 minutes, until the edges are set and the top is bubbly and lightly browned. A toothpick inserted into the crust near the edge should come out clean.

Rest and serve: Let the casserole rest for 10 minutes before slicing. Top with green onions or cilantro if you like.