Preheat oven to 350°F (175°C).

Grease a 9 x 18-inch baking dish.

Cook sausage in a skillet over medium heat until browned. Drain off any excess grease.

Stir in taco seasoning mix and cook according to package directions.

In a separate bowl, mix together sour cream, mayonnaise, 1 cup shredded cheese, and chopped onion.

In another bowl, combine Bisquick and cold water until a dough forms.

Press the Bisquick dough into the prepared baking dish, forming a crust.

Spread the cooked sausage and taco filling evenly over the crust.

Top with chopped tomatoes and green pepper.

Spread the sour cream mixture over the vegetables.

Sprinkle the remaining 1/2 cup of shredded cheese over the top.

Bake in the preheated oven for 35-40 minutes, or until the crust is golden brown and the cheese is melted and bubbly.

Let cool slightly before cutting into squares and serving.