Firstly grease a 9x13 casserole dish with nonstick cooking spray. Then, preheat the oven to 350 degrees Fahrenheit.
Brown ground beef and chopped onion in a large skillet over medium-high heat until no pink remains and onions have softened.
Add the taco seasoning to the meat mixture and stir well.
Then add Rotel tomatoes with all the juices and stir to combine. Set meat mixture aside.
Afterward mix together biscuit mix and water in a large bowl until a shaggy dough forms.
Then add the cheese and form a cohesive dough.
Afterward press the biscuit layer evenly into a glass baking dish.
Then add the ground beef mixture on top of the bisquick layer. Make sure to spread it out evenly.
In a separate bowl, stir together sour cream, mayonnaise, and 1 cup of grated cheese.
Then spread the sour cream mixture over the lean ground beef layer.
Top the casserole with the sliced bell peppers and the remaining cheese.
Bake in the preheated oven for 30-40 minutes, until casserole is bubbly and the cheese is melted.