Preheat a grill to high. Preheat the oven to 500 degrees F.
Shuck the oysters and set aside in the refrigerator or on ice, keeping them in the shell.
In a small bowl, combine the mayonnaise, white vinegar, Parmesan, salt and freshly ground black pepper, to taste, cayenne, and white pepper. Stir to combine, and then stir in the fried potato sticks.
In a small saute pan, add the olive oil and heat over medium-high heat. Add the bell peppers and red onions and saute for 3 minutes, and then add the spinach and allow to wilt. Adjust seasoning, to taste.
Top each oyster with the spinach mixture, dividing evenly, and then do the same with the mayo mixture. Top with the havarti and place on a baking sheet lined with a layer of rock salt. Roast in the oven until cheese is bubbly and oysters are just warmed through, 5 to 6 minutes.
Let cool enough to handle, and then serve right away.