Preheat the oven's broiler. Arrange the whole cloves of garlic, tomatillos, and jalapenos on a baking sheet. Place under the broiler, and cook for a few minutes. Remove garlic cloves first, as soon as they are toasted, to avoid developing a bitter flavor. Continue to roast jalapenos and tomatillos until evenly charred, turning occasionally. Set aside to cool. Don't remove the charred parts of the tomatillos or the peppers. They add a really nice flavor.
Place peppers and tomatillos in a blender with the garlic and cilantro. Add a little water to the mixture if necessary to facilitate blending. Season with salt and pepper to taste. Refrigerate until serving.