It amazed me, the first time I made these cubes and had them in the freezer, how often I found a use for them. They are so stinking convenient! Want to add a nice shot of mellow garlic flavor to soups, stews, sauces, etc.? Just pull a cube out, nuke it for ten seconds in the microwave and voila! You're good to go.
As I mentioned, I tend to roast about ten to twelve heads at a time. This usually lasts me a good while and I only have to go through this process once - maybe twice - a year. The first step is to preheat the oven to 400 degrees F. Then line a baking pan with foil. Cut the garlic heads in half so that each clove is exposed. Then put them, cut side up in the baking pan.
In my experience, based on how the garlic cloves are put together, its best to cut your bulb not quite in half. What I mean by that is to cut is so that the top "half" is larger. The bulbs are held firmly together on the bottom of the bulb, but this is not the case on the top. I've found by giving the top side a bit more substance, it holds together better. I also cut off the pointy top before I cut them in half as that allows the bulbs to sit more evenly in the pan.
Drizzle the heads with olive oil and close the foil tightly. Roast in the oven for 35-45 minutes at 400 degrees F or until the cloves are tender and slightly browned. Remove from the oven and let them sit to cool until they are easily handled. At this point, unless you are okay with your hands reeking of garlic for the next five years, you'll want to don some gloves. I keep a box of vinyl first aid gloves in the kitchen for this and other tasks. All you need do is squeeze the cloves out of their hulls. They should pop right out.
While you can do this next step by hand, it goes much faster in a food processor. Process the bulbs with just enough olive oil to make a nice, smooth paste.
Then spoon the paste into molds of some kind. You could use ice cube trays (just be sure they are designated "garlic freezing" trays as they will forever smell of garlic after this use), but I usually use mini-muffin tins. To make it easier to get the cubes out when frozen, I line the tins with plastic wrap, spoon in the mixture and then place more plastic wrap over the top. Freeze until frozen solid, pop out of the trays, and place into zip top bags for storage. Here's one last important tip: you'll want to double bag these suckers - they really have a potent aroma!!
Again, any time you're ready to use some, simply thaw in the microwave for a few seconds. Make these once and you'll be amazed how often you find yourself using them. And every time you come across a recipe that asks you to spend forty minutes roasting a single bulb of garlic, just smile to yourself and save thirty-nine minutes, forty-five seconds!
FYI: I've found that for mini-muffin tins, one cube equals about half a bulb of roasted garlic.