Position a rack in the center of the oven, and heat the oven to 350°F.
Rub the olive oil on the garlic, and wrap in aluminum foil. Bake until the garlic is soft, 30 to 45 minutes. Let cool slightly.
Using a serrated knife, halve the garlic crosswise. Squeeze all the cloves out into a small bowl.
In a 5- to 6-quart pot, melt 2 Tbs. of the butter over high heat. Add the garlic and cook until fragrant, about 30 seconds. Add the mussels and beer, cover, and cook until the mussels begin to open, about 4 minutes. Uncover, add the remaining butter and cilantro, if using, and cook until all the mussels are open, about 3 minutes more. Serve.
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