Bring a large pot of water to a boil for the gnocchi.
In a large skillet, heat the oil, one turn of the pan, over medium-high. Melt the butter into the oil. When the butter foams, add the sausage and walnuts. Cook, breaking up the sausage with a spoon, until the meat and nuts are browned, about 8 minutes. Add the shallots and garlic and stir until fragrant, another minute or two. Add the prosciutto and rosemary. Stir until the prosciutto is heated through, another minute or two. Add the brandy and stir to deglaze the pan. Add the cream and sage. Bring the sauce to a bubble, then reduce the heat to low.
Salt the boiling water. Add the gnocchi; cook until they float. Using a slotted spoon, transfer the gnocchi to the sauce; add the cheese and parsley and toss to coat. Serve in shallow bowls.