Pulse the cauliflower florets in small batches in the bowl of a food processor until the size of grains of rice.
Heat the oil in a large skillet. Add the leeks and a sprinkle of salt and saute until softened. Add the garlic and saute a few minutes more, until fragrant. Add the optional vegetables, first adding those that will take longer to cook, then any others.
Meanwhile, whisk together the tahini, nutritional yeast, miso paste and broth. When the leeks are tender and the other vegetables almost done, add the cauliflower. Cook for a few minutes more, then add the broth mixture and simmer for a few minutes more to thicken.
Remove from heat and stir in the lemon juice. Adjust the seasonings and serve garnished with parsley.