In a blender, whirl chiles, onion, and garlic into a smooth paste, adding 2 to 3 tablespoons water as needed to ease blending.
Add mint, cilantro, lime juice, cumin, and 1/2 teaspoon salt; whirl until smooth, adding 1 to 2 tablespoons water as needed to facilitate blending. Scrape into a bowl and add more salt to taste.
Ground toasted cumin: In a 6- to 8-inch frying pan over medium heat, stir 2 tablespoons cumin seeds often until lightly toasted, 2 to 4 minutes. Whirl in a blender until finely ground. Makes about 2 tablespoons.