Begin by bringing a large pot of salted water to a boil over high heat. Add elbow macaroni and cook until al dente, about 7-8 minutes. Drain and set aside.

In a large pot over medium heat, melt the unsalted butter until it bubbles gently. Whisk in the all-purpose flour, stirring continuously for 1-2 minutes until it forms a smooth paste.

Slowly pour in the whole milk, whisking constantly. Continue to whisk for about 2-3 minutes until the sauce thickens and bubbles gently.

Stir in the shredded Havarti cheese and sharp white cheddar cheese, mixing until both cheeses melt completely.

Add the drained macaroni back into the pot with the creamy cheese sauce, stirring until each piece is coated. Heat over low for 2-3 minutes.

If desired, fold in the frozen peas and let the mixture heat until the peas are warmed.

Serve immediately, garnished with pesto, basil, and optional red pepper flakes.