Step 1

Starting at cut end, grate tomatoes on the largest holes of a box grater into a medium bowl until all that’s left is the flattened tomato skins; discard skins. Set grated tomato aside.

Step 2

Heat vegetable oil in a medium pot over medium. Cook onions, stirring occasionally, until starting to soften, 5–7 minutes. Add garlic and cook, stirring often, until onions are very soft and evenly golden, about 5 minutes. Add ginger and pepper and cook until fragrant, about 20 seconds. Add grated tomato and cook, stirring, until thickened, about 3 minutes. Add star anise, cinnamon stick, and 5 cups water; season with salt. Bring to a simmer over medium-high, then reduce heat to medium-low and simmer gently until flavors come together, 8–10 minutes.

Step 3

Meanwhile, cook udon according to package directions. Drain and set aside.

Step 4

Add soy sauce and butter to broth and stir until butter is melted. Fish out star anise and cinnamon stick and discard. Season with more salt if needed.

Step 5

Divide noodles among bowls; ladle broth over. Add a few drops of sesame oil; scatter scallions and cilantro on top.