Preheat oven to 350ºF. Butter the bottom and sides of the casserole dish and dust with the breadcrumbs. Set aside. In a large sauté pan over low heat, sauté onions in olive oil very slowly, stirring frequently, until soft and translucent, about 20 to 30 minutes. Do this in two batches if needed. Remove the onions from the pan and add the sausage. Sauté the sausage until done. Drain, crumble, and set aside. Mix the ricotta, flour, eggs, cheese, and pepper. Add the cooked onions and sausage, and mix again. Fold in the spinach leaves. Fill the casserole dish with the mixture and bake for 60 to 90 minutes. (Cooking time will depend on the shape of your dish). If it begins to brown before it is cooked through, cover the top with foil and continue baking. Casserole is done when a cake tester inserted in the center comes out clean. Serve immediately.
