Preparation

• Light a charcoal fire or preheat your gas grill on high. Oil the grill’s cooking surface. Let the coals burn down to a medium-hot fire or adjust the gas grill burners to medium.
• Melt the butter in a large skillet over medium heat. Add the garlic and cook until you just smell it. Add the onions, celery, and bell peppers and cook, stirring, until soft, about 5 minutes. Stir in the spinach and milk,  and cook, stirring a few times, another 5 minutes. Remove from the heat.
• Shuck the oysters as if you were serving them on the half shell. Pour any liquor from the oysters into the spinach mixture. Place the oysters on a baking sheet. Add about 1 tablespoon of the spinach mixture to each oyster. (You may not use up all the spinach mixture.) Top with some cheese and breadcrumbs. Place the oysters on the grill, cover, and cook until the cheese is bubbling and beginning to brown, about 5 minutes.
•  Remove the oysters to a platter and serve with lemon wedges, if desired.