Pulse the onion, chopped bell pepper, tomato, cilantro, garlic and red pepper flakes in a food processor until finely chopped.
Heat the olive oil in a large Dutch oven or pot over medium-high heat. Add the chopped vegetable mixture and the potatoes and cook, stirring frequently, until the vegetable mixture looks dry, 10 to 12 minutes. Add the white wine and cook, stirring, until almost evaporated, about 2 minutes. Add 1 1/2 cups water and 1/4 teaspoon salt; bring to a simmer, cover and cook until the potatoes are tender, 8 to 10 minutes.
Add the mussels and sliced bell pepper and stir. Bring to a simmer, cover and cook until the mussels open, 5 to 7 minutes. (Discard any mussels that don't open.) Stir in the lemon juice and sprinkle with cilantro. Divide among bowls.
Per serving: Calories 309; Fat 12 g (Saturated 2 g); Cholesterol 54 mg; Sodium 677 mg; Carbohydrate 23 g; Fiber 2 g; Protein 25 g