For the soup: Melt the butter in a small saucepan over medium heat. Add the shallots and cook, stirring often, until soft and translucent, about 5 minutes. Add the tomato paste and cook stirring until the shallots are well coated. Add the diced tomatoes and 1 teaspoon salt, bring to a boil and reduce the heat to low. Simmer until the tomatoes have softened about 10 minutes.
Meanwhile, sprinkle the gelatin over 1/4 cup water and let sit until soft, about 5 minutes. Transfer the soup, gelatin and cream to a blender. Blend until smooth and pour in to a glass loaf pan. Chill until firm, about 1 hour.
For the dough: Put the flour in a large bowl. Pour half the hot water in and stir with a wooden spoon. Add the remaining hot water and stir until the dough starts to look shaggy. Add all of the cold water and oil. Keep stirring until the dough forms a ball.
Knead the dough on a floured surface until soft, smooth and springs back when you poke it with your finger, 8 to 10 minutes. (Alternately, you can knead the dough in a stand mixer with the hook attachment, on medium speed for 6 to 8 minutes.) Cover with plastic wrap and let sit at room temperature for 30 minutes.
Divide the dough into 4 equal pieces. Take 1 piece (cover the remaining 3 pieces with plastic wrap) and cut it into 4 pieces. Work with 1 small piece of dough at a time, keep the remaining covered with plastic wrap. Roll the dough between your palms to make a round, smooth ball. Using a rolling pin, roll into a 4 1/2-to-5-inch circle. (You may need more flour to keep the dough from sticking but try to use it sparingly.)
Fill the dough with 1 heaping tablespoon of tomato soup filling and then press 1 tomato half into it. Pleat the dumpling: Use your fingers to pleat and pinch the edge of the dough together, enclosing the filling, into a little satchel. Twist and pinch the tip of the dough close. Repeat with rest. Make sure that you cover any dough that you aren't currently using and cover the dumplings with a towel to prevent drying out.
Fill a large pot with 2-inches of water and insert a steamer basket that has a lid. Line the basket with a single layer of Napa cabbage leaves. Cover and steam over medium heat to warm up the steamer and soften cabbage, about 2 minutes. Place dumplings on the cabbage leaves, leaving 1-inch between each dumpling. Steam until the dough looks slightly translucent and the dumplings have slumped slightly, about 12 minutes. Serve in the steamer.
When measuring flour, we spoon into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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