In a medium-sized saucepan, heat olive oil.

Sauté onion over medium-high heat until golden, about five to seven minutes. Add tomato and sauté another two minutes. Lower heat.

Add the salsa, and cook for one minute until heated through. Add American cheese, shredded cheese, milk, and taco seasoning, and cook, stirring, until cheese is melted. Do not overcook, or the mixture will become clumpy.

Pour mixture over the pasta and stir to combine.