• Crumble or cut the tempeh into ¼-inch pieces.

In a medium [large] sauce pot, combine the tempeh and 1 cup [2 cups] water and bring to a boil. Cover and cook over high heat, stirring occasionally, until softened, 6 to 8 minutes. Drain the tempeh and transfer to a plate. Wipe the pot dry.
While the tempeh cooks, start preparing the vegetables.

• Peel and coarsely chop enough onion to measure ½ cup [1 cup] for the chili; finely chop as much of the remaining onion as you like for garnish.
• Remove the stem, ribs, and seeds from the bell pepper; coarsely chop the pepper.
• Rinse the black beans.

In the same pot used for the tempeh, warm 1 to 2 tablespoons oil over medium-high heat until hot but not smoking. Add the coarsely chopped onion, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Add the bell pepper and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Add the chili base, tempeh, beans, and ½ cup water and bring to a boil. Reduce to a simmer and cook, stirring occasionally, until the vegetables are tender and the chili has thickened, 5 to 7 minutes. Remove from the heat and season to taste with salt and pepper.
While the chili cooks, prepare the remaining ingredients.

• Cut away the core from the tomato; coarsely chop the tomato.
• Cut the melon in half lengthwise and scoop out the seeds. Cut the halves lengthwise into 2-inch-wide wedges. Season to taste with salt and pepper and as much sumac as you like.

Transfer the chili to individual bowls and garnish with the tomato and finely chopped onion. Serve the melon on the side.