Bring a large pot of water to a boil. Add 1 tablespoon of the salt to the macaroni and cook until tender but still firm to the bite, 7 to 8 minutes. Drain well.
Heat a large skillet over medium-high heat. Add the beef and onion and cook, stirring, until the beef is well browned. Carefully drain the fat from the skillet and stir in the tomato sauce, the remaining 1 teaspoon of salt, and pepper. Bring to a simmer over a low heat while preparing the remaining ingredients.
Combine the ricotta, sour cream, bell pepper, and scallions in a medium bowl. Spread half of the pasta into the bottom of a 9×13-inch baking dish. Top with the ricotta mixture, then the remaining pasta. Pour the meat mixture over the top. Sprinkle with the Cheddar and mozzarella. Bake the casserole until the cheese is melted and lightly browned, about 45 to 60 minutes, Or use a quick read thermometer to heat it to the desired temp. Sprinkle with the parsley before serving.