Tex mex mac n cheese

8 ounces uncooked penne pasta
1/4 cup butter or margarine
1/2 medium sweet onion, chopped
1 (8-ounce) loaf Mexican-style pasteurized prepared cheese product, cubed
1 (8-ounce) container sour cream
1 (10-ounce) can diced tomatoes and green chiles, drained

Cook pasta according to package directions, draining well.

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