In a large (10-inch) dry skillet, roast 4 heads of garlic (separated into cloves but not peeled) over medium heat, turning regularly until they’re soft and blackened in spots, 10 to 15 minutes. Cool, peel, place in a food processor and pulse until roughly chopped. Turn the machine on and add 2 cups olive oil in a steady stream. Stop the machine, add ¼ cup lime juice and ½ teaspoon salt and pulse to incorporate. Store refrigerated in a sealed container.