Roasting the vegetables first is a superb way to make a deeply flavored salsa.
Preheat oven to 450 degrees with rack in upper third. Put tomatoes, cut side up, on a rimmed baking sheet, along with onion, 2 jalapenos, and garlic. Drizzle with oil and roast until vegetables are blistered in places and wilting, 22 to 25 minutes. Remove from oven.
Pulse remaining jalapeno in a food processor until finely chopped. Add roasted vegetables and any accumulated juices, along with cilantro, salt, lime juice, and broth. Pulse until smooth.