Start by bringing a large pot of salted water to a rolling boil over high heat. Add pasta and cook according to package instructions, around 7-9 minutes, until al dente. Reserve a cup of pasta water, then drain and set aside.

Melt 2 tablespoons of butter in a separate saucepan over medium heat. Whisk in flour, stirring continuously for about 1 minute until it bubbles but does not brown.

Gradually pour in 1 cup of milk while whisking constantly. Cook for about 3-4 minutes until thickened, adjusting with reserved pasta water if necessary.

Once thickened, stir in ½ cup of pesto, mixing well and allowing it to heat through for 1-2 minutes.

Add drained pasta into the pan with the creamy pesto sauce, tossing gently to coat. Include any toppings like cooked chicken, shrimp, or spinach.

Taste and adjust seasoning with salt, then garnish with Parmesan cheese or toasted pine nuts before serving.