Preheat oven to 350 degrees F.
Cut top from each head of garlic, exposing the cloves. Place heads of garlic (cut side up), on a piece of heavy duty aluminum foil. Pour olive oil over them, and top with thyme springs. Season with salt and pepper. Wrap the foil tightly. Place in a small ovenproof pan, and bake until the cloves begin to pop out, about 1 hour. Remove from the oven and cool.
To remove the cloves, open the foil and squeeze the lower part of the head of garlic. In a small bowl, mash the cloves to form a paste. (At this point the paste can be used or stored in the refrigerator or freezer.)
Add butter and chopped thyme to the bowl, stirring to combine. Season with salt and pepper, to taste.
Toast both sides of the bread, using a hot grill, grill pan, or broiler. Spread the roasted garlic butter paste onto the toasted bread. Serve immediately.