Preheat your oven to 350°F and lightly oil a casserole dish.
Place the cauliflower in a large pot. Fill it with enough water to cover the vegetables plus about two inches.
Bring the water to a boil over medium-high heat and then reduce the heat and allow to simmer for about 15 minutes or until the vegetables are fork tender.
Drain the cauliflower and allow them to cool slightly and then place them in a high-speed blender or food processor along with the vegetable stock, lemon juice, nutritional yeast, Dijon mustard, garlic, garlic powder, onion powder, salt, and black pepper and blend until smooth and creamy.
In a casserole dish, layer the potatoes and pour the cauliflower cream over it. Sprinkle the top with the fresh rosemary.
Cover with foil and cook for 30 minutes. Remove the foil and cook for another 20-30 minutes, or until the potatoes have browned and the sauce is bubbly.