Preheat a grill on high heat. Heat butter and oil in a large saucepan over high heat, add leek and stir occasionally until very soft and starting to caramelise (5-10 minutes). Add garlic and cook until just fragrant (1 minute), then add chicken stock and bring to the boil.
Meanwhile, grill bread slices until golden, turning once (1-2 minutes each side). Top toast with cheeses and grill until golden and bubbling (2-4 minutes).
Ladle soup among bowls, top with a toast and oregano leaves and serve immediately.