1) Start cooking pasta as directed.
2) Throw chopped veggies (except garlic) in a sauce pan and add water to just cover. Cover with lid and boil until tender.
3) In a food processor, add garlic, cashews, lemon juice, mustard, margarine, and salt and 1/2 cup of broth from the veggies. Process until very smooth, adding a bit more water if it's too stiff and starts to ball up (should resemble the consistency of creamy peanut butter). Add veggies by scooping them out with a hand-held strainer, and process until smooth, adding more salt if needed, or more broth if it needs to be thinned (should look like a thick gravy). You can also play around with spices that you like.
4) Pour mixture into a medium sized casserole dish, and dump in cooked pasta. Mix evenly to coat.
Note: The original recipe baked the macaroni mixture for another 30 minutes with breadcrumbs on top. I prefer mine creamier, so I skip this part (plus I'm a bit impatient and it taste incredible as is).
Source of recipe: An adaptation of Allison Rivers Samson's recipe in Veg News Magazine posted here: http://www.vegnews.com/web/articles/page.do?pageId=40&catId=10 [1]. This is the best recipe I've come across so far, and pretty versatile. You can use other veggies to make different creamy pasta sauces, like zucchinni & spinach- just make sure you have the potato for the consistency and try to match colors so you don't end up with some weird brown sauce! This combination is surprisingly cheesy, even without nutritional yeast or processed vegan cheeses (which you'll find in most mac 'n' cheese recipes). And, it has a surprisingly cheesy texture due to the potato- even taste good cold!