Cook pasta in salted boiling water according to package direction. Drain and set aside.
Melt butter in a Dutch Oven or large skillet. Saute onion in melted butter over medium heat until onions are translucent. Add garlic and cook for 30 seconds.
Whisk flour into butter mixture; cook for 1 minute. Add milk, bouillon, salt, cumin and chili powder. Cook over medium-low heat until sauce has thickened, stirring frequently.
Fold green chilies and diced tomatoes into sauce. Remove from heat and stir in 2 cups cheese and sour cream; stir until cheese is melted.
Add shredded chicken and pasta, tossing until pasta is coated in sauce. If desired, top with ½ cup shredded cheese and pop in oven at 350°F for 10-15 minutes, or until cheese is melted.