Thaw and drain 1/2 lb. frozen spinach, squeezing out excess water. Set aside.

Boil 2 cups macaroni for 7-10 minutes until tender. Drain well.

Melt 2 Tbsp butter, then whisk in 2 Tbsp flour for 1 minute to form a paste.

Gradually whisk in 2 cups milk, simmer until thickened, then remove from heat.

Stir in 1/2 cup Parmesan, 1/4 cup pesto, 1/2 tsp salt, and cracked pepper until smooth.

Combine cooked macaroni, drained spinach, and sauce in the pot. Stir to coat and serve immediately.