1. Preheat broiler. Slice baguette in half lengthwise and place on a baking sheet cut side up; spread cut sides with herbed butter spread. Transfer baking sheet to broiler and broil until tops are toasted, 2 to 3 minutes. Turn and continue broiling until bottoms are toasted, 2 to 3 minutes. Let bread cool to room temperature.

2. Place bell peppers directly over the flame of a gas-stove burner on high heat or on a grill. Roast peppers, turning with tongs, until blackened all over. (Alternatively, broil peppers on a baking sheet, turning, until skin has charred.) Transfer peppers to a large bowl, and cover immediately with plastic wrap. Set aside to steam, about 15 minutes. Peel peppers; discard skins. Remove stems, seeds, and ribs, and chop. Set aside.

3. Place olives in the bowl of a small food processor; pulse until finely chopped. Add anchovies and pulse to combine. Scrape down sides of processor and add olive oil and peppers; pulse to combine.

4. Divide olive mixture evenly between baguette halves, spreading to cover. Top each half with prosciutto slices and garnish with cheese; serve.