Cut garlic heads in half across the cloves to expose the insides. Place on a large piece of tin foil. Drizzle with olive oil, salt, and black pepper. Wrap up the foil around the garlic to make an enclosed pouch.
Roast the garlic until turning brown and soft, approximately 1 hour.
Remove the garlic cloves from the skins by squeezing at the base and place the cloves in a blender.
Add the balsamic, water, honey, mustard, salt, and black pepper to the blender and blend to combine.
While the blender is running, pour in the olive oil slowly to incorporate, approximately 1 minute.
Season to taste with additional salt and pepper. Thin if needed with more water. Store refrigerated up to a week.
A food processor can also work in place of a blender.