Step 1In a large pan, bring to a boil enough salted water to cover the potatoes. Add the potatoes and cook until tender, 15 to 20 minutes.
Step 2Drain the potatoes, then press through a ricer. Cover the potatoes and keep them warm while preparing the sorrel.
Step 3Heat 2 tablespoons of the butter in a skillet over medium heat. Add the onion and saute until tender, 3 to 4 minutes. Add the minced garlic and continue to cook for 1 minute.
Step 4Add the remaining 1 tablespoon of butter and heat until melted. Stir in the the sorrel and heat until it begins to wilt, about 30 seconds to 1 minute. Stir into the potatoes. Stir in the lemon peel, cracked black pepper and salt to taste.
Step 5Shape into 10 small cakes. Combine the beaten egg with the water. Dip the cakes into egg mixture, then into the bread crumbs to coat both sides.
Step 6Heat the oil in a large skillet over medium heat. Fry the cakes until browned, 4 to 5 minutes, then turn to brown the other side, 3 to 4 minutes. Serve with smoked salmon or lox, chicken or poached eggs.