Add the basil, garlic, Parmesan or Pecorino Romano, walnuts and a generous pinch of kosher salt to a food processor.

Pulse approximately 10 times to begin breaking down the ingredients.

With the food processor running, slowly stream in the olive oil until the pesto reaches your desired consistency. I usually use about ½ cup and add more as needed when I'm preparing a dish.

Use immediately or store in the refrigerator for 3–5 days. Pesto can also be frozen so it's ready for a quick meal later.

Slice the cooked chicken breast and place it in a bowl. Add enough pesto to thoroughly coat the chicken and stir to combine.

Cut a sandwich-size portion from the French loaf and slice it open.

Remove some of the soft bread from the center of each half to make room for the filling.

Spread mayonnaise over the inside of both pieces of bread.

Add 2 slices of Havarti cheese, or substitute your favorite cheese.

Add as many slices of pesto-coated chicken as will comfortably fit and close the sandwich.

Place the sandwich in a preheated panini press and cook until the bread is golden and crisp and the cheese is melted. Serve immediately.