There are reasons that classic combinations endure... they are delicious, and leek and potato soup is a case in point. gently fry 2 chopped, and washed leeks, in olive oil, add crushed garlic and 5(ish) chopped potatoes and a chopped carrot. Stir around in the oil briefly and add mustard - English powdered is great. Add stock / stock and water and simmer for 15-20 minutes. Zizz, taste and season ... I think a good grinding of black pepper is essential. Scoff... go ahhhhhhh, like in the adverts... curl up on sofa, preferably in front of roaring fire.