Preheat the oven to 300 degrees F.
Bring a medium saucepan with salted water to a boil. Cut the garlic heads in half crosswise and blanch for 5 minutes. Remove the garlic heads and any loose garlic cloves from the water with a slotted spoon and drain well. Pour olive oil into a small baking dish and place the garlic heads cut side down into the oil. Add any loose garlic cloves and the onion quarter to the oil. Cover the dish tightly with aluminum foil and bake for 1 hour.
Remove the garlic from the oil and squeeze the softened garlic cloves into a medium bowl. Peel the onion quarter and finely dice. Add the diced onion to the garlic cloves and mash with a fork until smooth. Stir the Worcestershire and lemon juice into the mashed garlic and continue stirring until smooth and creamy. Season with 1/4 teaspoon of salt. The garlic jam will keep refrigerated in a sealed container for up to 2 weeks.
Yield: about 1/2 cup