||Preheat oven to 180C. Heat oil and half the butter in a saucepan over medium heat, add onion, garlic and potato, cover, stir occasionally until onion is tender (5-7 minutes). Add cauliflower and stock, simmer over medium-high heat until potato and cauliflower are very tender (20-25 minutes). Purée until smooth, stir through Gruyère, milk and remaining butter, season to taste, keep warm.
||Meanwhile, for mustard and Gruyère croûtons, combine ingredients in a bowl, season to taste, toss to coat. Spread on an oven tray lined with baking paper and bake, shaking occasionally, until golden (5-7 minutes). Scatter over hot soup, serve drizzled with oil.