Preheat oven to 375 degrees.
Make the herby pesto by adding all of the ingredients, except the olive oil, to a small food processor. Process until thick and a bit chunky. Slowly add the olive oil and process until combined. Add salt and cracked pepper to taste. Set aside.
Prepare the pasta al dente, according to package instructions. Drain and add the cooked pasta to a shallow casserole dish.
Melt the butter in a saucepan over medium heat.
Add the flour and whisk to create a paste.
Slowly add the milk, whisking until combined. Cook for about 8 minutes, whisking until the sauce begins to thicken. Season with salt and pepper.
Remove from heat and add 2 C sharp white cheddar and 1/2 C Parmesan, whisk until melted and thickened.
Pour the cheese sauce over the cooked pasta and mix to combine.
Add dollops of the herby pesto to the top of the pasta mixture and swirl through to incorporate.
Sprinkle the remaining sharp cheddar and Parmesan on top. Cover with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10 - 15 minutes until the cheese is bubbling and slightly browned.
This recipe was inspired by Kathy Gunst’s Summer Green Mac and Cheese recipe.