Cut the top off the garlic head to just expose the bulb. Remove any excess papery skins.
Place in a large sheet of aluminum foil. Drizzle with 1 tablespoon olive oil. Season with salt and pepper. Close the foil around the bulb, leaving a space between the top and the foil. Place onto a baking sheet and into the oven. Roast the garlic until soft and fragrant, about 1 hour.
Remove from the oven and cool completely. Gently squeeze the cloves into a dish. (Can be prepared in advance and stored in an airtight container for up to a week.)
Set a large saute pan over medium heat and add the remaining tablespoon oil. Once hot, add the onions and season lightly with salt and pepper. Brown until evenly caramelized, 5 to 7 minutes. Deglaze with the sherry vinegar, white wine and saffron. Stir to scrape up any browned bits. Reduce the heat to medium-low, and stir in the roasted garlic. Cook for an additional 10 minutes, stirring occasionally, until the jam is thickened and glossy. Remove from the heat and cool. Season with salt and pepper.