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.