Cook the pasta:
Bring a pot of salted water to a boil. Cook the pasta according to package directions until al dente. Drain and set aside (do not rinse).

Steam the veggies:
Add 1½ cups water to a large saucepan and place a steamer basket inside. Add the cauliflower florets and sweet potato cubes. Cover and steam over medium heat for 8–10 minutes, or until fork tender.

Make the veggie puree:
Transfer the steamed cauliflower and sweet potatoes to a blender. Add ½ cup of milk and blend until completely smooth and creamy.

Prepare the roux:
In a large pot over medium heat, melt ghee or butter. Whisk in the arrowroot starch or flour until a smooth paste forms. Slowly whisk in the remaining ¾ cup milk and continue to stir until the mixture starts to bubble and thicken.

Build the cheese sauce:
Reduce the heat to low. Stir in shredded cheddar, veggie puree, garlic powder, Dijon mustard, sea salt, and black pepper. Mix until the cheese is fully melted and the sauce is smooth.

Combine:
Add the cooked pasta to the cheese sauce and mix until evenly coated. Taste and adjust salt if needed.

Optional broil:
Transfer to a baking dish and broil for 5 minutes to create a golden, crispy top.