1. Preheat the oven to 300 degrees. Halve the garlic heads crosswise. In a saucepan of boiling salted water, blanch the garlic for 5 minutes; drain well. Trim off the roots.
2. Pour the oil into a 9-by-13-inch baking pan. Add the garlic, cut side down, and roast for 1 hour, or until the garlic is soft.
3. Squeeze the garlic from the skins into a coarse strainer set over a bowl. Press the garlic through the strainer with a spatula. Whisk the puree until creamy.
The jam can be refrigerated for up to 10 days.
Save the garlicky oil to use in cooking.