Remove mushroom stems and chop caps. Peel onions, remove green tops and roots and chop finely. Combine mushrooms, onion, minced pork, eggs, soy sauce, sugar and mirin in a large mixing bowl. Season with salt and knead to a smooth paste. BLend in half the breadcrumbs, knead again, then form mixture into 1 1/2 inch balls. Roll meatballs in remaining breadcrumbs, pressing on lightly. Heat 1 inch oil in a wok and fry meatballs until golden brown and cooked through. Drain and keep warm. For sauce, combine sake or wine, soy sauce, sugar, stock and chili powder in a small saucepan and bring to a boil. Cool, add lemon juice and transfer to small sauce bowls. Thread several meatballs on each skewer. Line a serving dish with lettuce and arrange skewers on top. Serve with the sauce.