In a stock pot, over medium heat, stir the oil and flour together. Cook, stirring constantly, for 10 to 15 minutes for a blond roux. Add the onions. Season with salt and pepper. Cook for 8 minutes. Stir in the garlic and mushrooms. Season with salt and pepper. Cook for 4 to 6 minutes or until the mushrooms wilt. In a large hot saute pan, render the foie gras quickly for 1 to 2 minutes. Remove from the heat and add to the mushroom mixture, including the foie fat. Add the beer and stock. Bring the liquid to a boil and reduce to a simmer. Cook for about 1 hour. Using a hand-held blender, puree the soup until smooth. Whisk in the cream. Season with salt and pepper. Serve with the croutons.