Heat the oven to 325 degrees. Break the heads of garlic apart, then mash each clove (a fist against the side of a knife is what I do) to release the clove from its papery skin; if using already-peeled garlic, scoop the cloves into a heavy plastic bag and use a rolling pin to mash them slightly.
Stir together the garlic, oil and salt in an 8x8-inch baking pan (make sure all the garlic is submerged), slide it into the oven and bake until the garlic is soft and lightly brown, about 45 to 55 minutes.
Add the lime juice and return to the oven for 20 minutes for the garlic to absorb the lime and turn golden brown. (If you’re using the larger quantity of oil, ladle off 1 cup—no garlic cloves—and store it in a cool dry place for use in salad dressing or sautéing.)
Using an old-fashioned potato masher or large fork, mash the garlic into a coarse puree. Pour the mixture into a wide-mouth storage container and refrigerate it until you’re ready to enjoy some deliciousness. The mojo will last for up to three months as long as the garlic stays submerged under the oil.