Heat the oil and butter in a skillet over medium-low heat. Once hot, add the onions and sauté until translucent.
Squeeze the juice from the lemon over the diced apple.
Add the diced apple and Panko to the skillet. Mix thoroughly until the bread crumbs are no longer dry, then remove from heat.
Add the cardamom and pepper. Stir well and set aside.
Preheat the oven to 450°F. Line a baking sheet with a thin layer of rock salt.
Shuck the oysters, cup-side down, reserving as much liquid as possible in the cup.
Place the oysters on the cookie sheet, using the rock salt to keep them from tipping over.
Divide the apple mixture between the oysters.
Place the baking sheet in the oven and roast the oysters for about 10 minutes.