Boil and drain pasta according to package directions. Place the cooked pasta in a 9x13 baking dish.
While the pasta is cooking, make the pesto sauce: Add all ingredients to a blender or food processor. Puree until smooth.
Preheat oven to 375 degrees.
Melt butter over medium-low heat in a large saucepan. Add the garlic and flour, and whisk until bubbly.
Pour in the milk. Bring to a boil, whisking occasionally. Lower the heat and stir in the mozzarella and pesto sauce. Stir until smooth. Pour the mixture evenly over the cooked pasta.