Heat 4 tablespoons butter in a large skillet over medium heat. Add the onions, chopped anchovies, thyme, salt and pepper and cook until the onions are sweet and golden brown, about 20 minutes.
Meanwhile, holding the knife parallel to the cutting board, shave off the rounded top and bottom of the baguette with a serrated knife so that it lies flat. Then cut the baguette in half lengthwise to make 2 long slices of bread. Brush both sides of each with the remaining 4 tablespoons butter.
When the onions are cooked, put the bread slices on a baking sheet and spread with the onions. Garnish with olives and drizzle with olive oil. Place in the oven and bake until the bread is crispy, about 15 minutes. Shower with Parmigiano. Cut into pieces and serve hot.
Serve the onion toasts alongside an assortment of Italian meats.