Step 1Heat 4 tablespoons of olive oil in a large skillet over medium heat. Add the prosciutto and cook until soft, 5 minutes. Add the onions and cook 3 to 5 minutes. Add the garlic and cook about 1 minute, then add the tomatoes and peppers.
Step 2When the tomatoes begin to melt, add the white wine and anchovies; reduce the heat. Cook until the liquid has evaporated, about 30 minutes. Season with salt and pepper.
Step 3To clean the sardines, cut the head at the top and the bottom just behind the gills, cutting to but not through the spine. Grasp the head and give it a twist to snap the bone, then pull gently to remove it. Most of the entrails will come away with the head. Discard these. Using the point of a sharp knife or kitchen scissors, slit the sardine open along the belly and rinse it well. Still using just the tip of the knife, cut through the fine ribs on either side of the spine without cutting all the way down through the skin. Grasp the spine and gently work it free down to the tail. Snap the bone at the tail and discard the spine. Cut off the fins and remove any fine bones. Repeat with the remaining filets. The fish will be served flesh-side down, so that side can look less than perfect.
Step 4Pat the filets dry and neaten them up, trimming the tops and the belly flaps to make rectangles. Season with salt and pepper on both sides. Brush the skin sides with mustard, then sprinkle them with breadcrumbs. Pat to fix the crumbs in place.
Step 5Heat the remaining 2 tablespoons of olive oil in a large nonstick skillet over high heat. When the oil is almost smoking, add the sardines, skin-side down and reduce the heat to medium. Cook the sardines until you see the meat on the top of the flesh side begin to turn color, 4 to 6 minutes.
Step 6While the sardines are cooking, reheat the tomatoes and peppers . Just before serving, add the basil and mix well.
Step 7Spoon the tomato-pepper mixture in equal portions onto six small plates. Lift the sardines from the skillet with a slotted spoon and place 2 of them skin-side up on each mound of the tomato-pepper mixture. Serve immediately.