1. Place peas, mint, 2 tablespoons water, oil, 1/4 teaspoon pepper, and 1/8 teaspoon salt in the bowl of a food processor; process until smooth.
2. Hollow out bottom and top halves of bread, leaving a 3/4-inch-thick shell; reserve torn bread for another use. Cut bread halves in half crosswise. Spread pea mixture evenly over cut sides of bread; top evenly with prosciutto, onion, and tomatoes. Sprinkle with remaining 1/4 teaspoon pepper, remaining 1/8 teaspoon salt, and goat cheese.