Pre-heat oven to 400° F.
While the oven heats up, remove the fish fillets from the refrigerator, pat dry with a paper towel, and let them come to room temp.
Lightly grease/oil a cast iron skillet or baking dish.
Mix the pesto, chopped walnuts, Greek yogurt, and grated parmesan/romano in a small bowl.
Before coating the fish with the pesto mixture, give each fillet a little spritz of lemon juice, and then hit the fillets with a bit of kosher salt and ground black pepper. Coat the tops of the fillets evenly with the pesto mixture.
Bake fish for 13 minutes or until fish is firm to the touch and pesto walnut crust is starting to lightly brown. An alternative way to do this is to bake the fish for 11 minutes and put it under the broiler (on high) for 2-3 minutes. Keep a close eye on the fish while broiling since all broilers are different.