Cut the heads of garlic almost in half horizontally, keeping the heads mostly intact. Set them in the center of a large piece of aluminum foil and drizzle generously inside and out with oil and sprinkle with salt. Tuck a rosemary sprig or two in with the garlic and fold the edges of the foil together to make a packet. Put in the slow cooker. Cover and cook on low for 3 to 4 hours, until the garlic is very soft.
Unwrap, squeeze the garlic cloves into a small airtight container, cover with more oil, if you’d like, and refrigerate, covered, for up to 1 week or freeze for up to 6 months.
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