Mix in a blender the soaked cashews, water, nutritional yeast, lemon juice and miso paste. Blend all the ingredients together until smooth consistency.
Heat 1 teaspoon of olive oil into low-medium heat and add the leek,mushrooms, and nutmeg. Cook for a few minutes until the leek has softened and the mushrooms are fully cooked. If it sticks to the pan, instead of more oil you can add a little bit of water.
Add the blended mixture with the leek and mushrooms and cook for a few more minutes.
Transfer the sauce into the blender again, and blend for a few seconds. You do not need a really smooth sauce, we like it better when it still has some bits that Luca can pick and eat. Adjust the consistency of the sauce to your toddler or kid's taste.
Boil the pasta following the package instructions, rinse with cold water and add to the sauce.