Prep the veggies: Slice the leek lengthwise, and rinse it under cold water. Finely chop the white and light green parts and save the dark green parts to make vegetable broth. For the onion, cut off the ends, slice it in half, and slice it finely.
Sauté the leeks and onions: Heat olive oil in a large frying pan over medium-high heat. Add the leeks and onions and sauté for 3 minutes until soft. Season with salt and pepper.
Caramelize the veggies: Lower the heat to medium and let the vegetables caramelize for 20 minutes, stirring every 5 minutes to prevent sticking.
Cook the pasta: When the leek is almost done, cook the pasta according to package directions.
Make the sauce: Stir the cashew butter and vegetable broth into the pan with the leek. Add a squeeze of lemon juice, sprinkle in the nutritional yeast (if using), and mix until the sauce is creamy and thickened. Taste and adjust seasoning with salt, pepper, or more lemon juice if needed.
Toss with pasta: Add the cooked pasta to the pan, toss everything together, and serve.