Preheat the oven to 450 degrees F. Prepare a baking dish with a generous layer of rock salt.
Set a large skillet over medium heat, add the butter and let it melt. Add the olive oil, garlic and shallots and saute until fragrant, about 2 minutes. Add the chopped spinach and toss with the garlic and shallots. Add the Parmesan, anise-flavored liqueur, breadcrumbs and cayenne pepper. Season with salt and pepper and cook for another 3 minutes.
Spoon about a tablespoon of the spinach mixture onto each oyster and nestle the oysters into the rock salt. Bake until the topping is crisp and brown, about 10 minutes. Serve with lemon wedges.
Use any leftover spinach mixture as a filling for an omelet.