Cook the pasta according to the package directions, draining well. Melt the butter in a large skillet over medium heat, then add the onion and sauté for 3 to 5 minutes or until tender. Add the cheese and cook, stirring constantly for 5 minutes or until the cheese melts. Stir in the sour cream, blending well. Stir in the tomatoes, green chiles and pasta and cook, stirring constantly for 5 minutes or until thoroughly heated.