To make the stuffing, put the raisins, olives, pine nuts, parsley, and garlic on a cutting board and chop, and put into a mixing bowl. Add the sausage and 1/2 cup of the cheese. Drizzle the stuffing with about 1/4 cup of olive oil to moisten and season with salt, pepper, and red pepper flakes; mix well with your hands to combine. Put the bread crumbs in another bowl, drizzle with a bit of olive oil and add 1/4 cup of the Pecorino; season with salt and pepper.
Oil the bottom and sides of a casserole dish which is large enough to hold all the mushrooms in a single layer. Arrange the mushrooms snugly in the pan, cap side down; season the insides with salt and pepper and drizzle with olive oil. Stuff each mushroom with a generous spoonful of the sausage stuffing and sprinkle the bread crumbs on top; drizzle with even more oil. Bake for 20 minutes until the stuffing is browned and the mushrooms soft, rotating the dish periodically for even cooking.