Make a "dish" from double layered or heavy-duty foil. It should be just large enough to contain the fillets. Place fish in foil dish and spread butter bits evenly over top of fish. Juice two lemons atop fish, catching seeds through a small sieve.
Grill fish on both sides until cooked through. Leave grill lid open, or close for quicker cooking.
While fish is grilling, prepare the sauce. Melt butter in a saucepan and add garlic, mustard and lemon zest. Whisk in the pan until blended and continue stirring for about 2 minutes over medium heat.
Serve each fillet with a generous dollop of sauce.
Notes
If you're preparing this recipe as gluten-free, just be sure to use a brand of Dijon that is known to be GF.