Prepare a grill fire to 300° with pecan or hickory for smoke flavor.
Pat fish and shrimp dry with paper towels and layer on a baking sheet.
In a saucepan, combine butter, Parmesan cheese, lemon zest, lemon juice, dijon, honey, garlic, thyme, salt, pepper and chili flakes if desired.
Cook over low heat for about five minutes, whisking until well combined. Set aside to cool slightly.
Reserve about 1/4 cup of sauce for serving.
Brush sauce over fish and shrimp.
Cook over indirect heat for 5-8 minutes with grill closed.
Check internal temperature of fish and shrimp and continue to cook until it reaches 145° for both.
Remove from grill and brush with remaining sauce.
Garnish with parsley and serve immediately.