Heat the stock to a simmer and remove it from heat. Heat a large pan over medium heat. Add the oil, 2 tbsp butter, and bone marrow and allow them to melt. Add the onions and cook until soft. Add the garlic and cook for 1 minute. Add the rice and toast for 2-3 minutes. Add the white wine. Start adding the stock 1 1/2 cups at a time adding more when the liquid is almost absorbed. Stir frequently. Allow the last addition of stock to absorb for 2-3 minutes. Then, add the parmesan and the remaining butter. Season with salt and pepper to taste. Serve immediately.