In a saucepan over medium-high heat, combine oil and butter. Add onions and leeks and cook until caramelized, about 10 to 15 min. Add beer, broth, potatoes, 1/8 tsp (0.5 mL) salt and pepper and bring to a boil.
Reduce heat to medium and simmer 20 min. or until potatoes are tender.
Transfer broth mixture to a blender and purée until soup is smooth. Adjust seasoning, if desired. Divide among 8 bowls and garnish each with crème fraîche and one toast.