Place the first 3 ingredients in a food processor, and process until minced. Add lime juice, fish sauce, and tofu; pulse until blended. Heat oil in a small skillet over medium heat; sauté ginger and garlic 2 minutes. Add to shrimp mixture, and pulse until combined. Add onions and chopped cilantro, and pulse 3 to 4 times. Spoon into a bowl; cover and chill 1 hour. Garnish dip with additional shrimp and cilantro sprig, if desired.