Preheat the oven to 400ºF. Lightly butter a medium casserole dish. Season the tilapia with salt and pepper. Add 1 tablespoon of the parsley, lemon juice, horseradish, mustard, and broth to the casserole dish and whisk to combine. Place the tilapia fillets in the casserole dish and spoon the horseradish mixture of them. Top each fillet with a slice of butter and bake, basting occasionally if desired, until the fish flakes easily with a fork, about 20 minutes. Sprinkle the fish with the remaining 2 tablespoons of parsley and lemon zest. Serve with the lemon wedges.