Move oven rack to top position and preheat broiler.
Place 1 lb halved Roma tomatoes skin side up, 1/2 medium white onion sliced, 1/2 medium red onion sliced, and 2 unpeeled garlic cloves on a rimmed metal sheet pan.
Broil for 6-8 minutes until tomato skins are blackened and blistered and onions are charred at edges.
Cool until garlic is safe to handle, then peel garlic cloves.
Add charred tomatoes, onions, peeled garlic, 1 chipotle pepper in adobo, 1/2 tsp adobo sauce, 1/2 tsp kosher salt, 1/4 tsp black pepper, 1/4 cup fresh cilantro, and juice of 1/2 medium lime to food processor.
Pulse until desired consistency is reached.
Taste and adjust salt if needed.
Refrigerate for 30 minutes before serving for best flavor.