Cook pasta to al dente, according to package directions; drain, then set aside in a large mixing bowl.
Mince garlic; peel and remove the pits from the avocados, scooping the flesh into a blender or food processor. Add in the garlic, lime juice, cilantro, salt, and pepper; purée until smooth, then cover and set aside.
Heat butter in a small saucepan over medium heat.
Once butter has melted, add in flour and whisk together to make a paste-like consistency. Add milk and continue whisking constantly until smooth and thickened.
Add cheese and stir until smooth.
Add avocado puree to pasta and stir until combined. Add in the cheese sauce and stir until fully coated.