Combine 4 tablespoons of the butter, the shallots, garlic, bacon and salt in a large saute pan over medium-high heat. Saute until the shallots are translucent and the bacon is crispy, 6 to 8 minutes. Add the flour and mix well to make a roux. Cook, stirring frequently, until the roux smells nutty, 4 to 5 minutes. Off the heat, deglaze the roux with the liqueur; carefully return the pan to the burner and let the liqueur flame. Add the cream and stir to incorporate. Bring the mixture to a boil, then reduce to a simmer and cook for 5 to 7 minutes. Stir in the baby spinach, nutmeg, black pepper, and red pepper flakes if using; bring back to a boil and once again immediately reduce to a simmer. Cook until the spinach has softened and released its liquid, 4 to 6 minutes. Over medium heat, cook the mixture down to a fondue-like thickness, 6 to 8 minutes. Remove from the heat and reserve to the side.
Melt the remaining 4 tablespoons butter in a small saute pan. When melted, remove from the heat and incorporate the breadcrumbs. Reserve to the side.
Heat a grill to high heat. Preheat a broiler.
Place the oysters on the grill and cook until the shells pop open, about 3 minutes. Remove the oysters to a cutting board and use an oyster knife to pry out the top shells, scraping the knife around the inside of each top shell to loosen the oyster. Once the top shell has been removed, discard it; then use the knife to scrape under the bottom of the oyster to separate it from the bottom shell, leaving the oyster "on the half shell."
Spread the rock salt evenly on a baking sheet. Set the oysters, shell down, on the rock salt, pressing the shells into the salt. Spoon some of the baby spinach bacon fondue on top of each oyster; then sprinkle with the Parmesan, Gruyere and, lastly, the buttered breadcrumbs. Broil until the oysters are bubbly and the breadcrumbs toasted, 4 to 5 minutes. Serve immediately.
Use extreme caution when igniting alcohol. Remove the pan from the heat source before adding the alcohol. Pour the alcohol into the pan and carefully ignite with a match or click lighter. Return the pan to the heat and gently swirl to reduce the flames.