4: Pour 5 tablespoons olive oil into a large saucepan on high heat. When really hot, add the chopped onion. Salt and pepper, then cook for 1 minute without colouring.
5: Add the pieces of celeriac and potimarron and cook for 2 or 3 minutes, stirring frequently.
6: Add 1 litre whole milk to completely cover the vegetables (adjust the quantity as necessary). Salt and pepper, then leave on low heat to simmer until the vegetables are soft.
7: When the vegetables are cooked, add 1 tablespoon Mustard and blend until the soup is smooth.
You will see in the photo for stage 2 that I have used a green variety of potimarron, but the more usual orange sort will work just as well. You can also use ordinary pumpkin, though this needs to be peeled. The proportions stated here (1 potimarron, 1 celeriac, 1 onion) are only for guidance, so do feel free to adapt them.