For the dumplings: Combine the flour, 1/2 teaspoon salt and 1 cup water in a bowl. Mix with your fingers until the water is incorporated and a shaggy dough forms. Do not overwork. Form into a log and roll between your hands until somewhat smooth. Set aside to rest.
Bring a pot of water to a boil. Add a pinch of salt and a little bit of oil. Pull a golf ball-size piece of dough from the log. Form into a ball, then flatten into a disc with the palm of your hand. The center should have a dent and be slightly thinner than the edges. Repeat with the remaining dough to make more dumplings. Drop the dumplings into the water and simmer until the dumplings float to the surface, 30 to 40 minutes. Transfer to a plate with a slotted spoon.
For the mackerel: While the dumplings are cooking, melt the butter in a large skillet over medium-high heat. Add the thyme, chiles, onions, bell peppers, scallions and tomatoes and sprinkle with black pepper and all-purpose seasoning. Cook, stirring often, until the vegetables are tender, 2 to 3 minutes. Stir in the mackerel and its sauce and cook until heated through, about 3 more minutes.
Serve the mackerel with the dumplings.
NotesThis recipe was provided by a chef, restaurant or culinary professional and may have been scaled down from a bulk recipe. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.