Preheat the broiler to high, placing the rack in the upper third of the oven. Line a baking sheet with tin foil and toss, directly on the sheet, the tomatoes and onion with the olive oil. Broil, stirring now and then, until the tomatoes have collapsed and the onions have softened a bit and everything is slightly charred, about 10 minutes. Place the tomatoes and onions in a food processor with the chipotle, cilantro and lime juice, and blitz until everything is pured. Remove the salsa to a bowl and season to taste with salt.