Preheat oven to 390°F (200°C).
If you have time, soak the cashews in water overnight and drain. For a quicker version, boil them for 15 minutes, then rinse under cold water and drain.
Tip: If your blender isn’t high-powered, boil the cashews for 25 minutes to ensure a silky-smooth sauce.
Toss the cauliflower florets and unpeeled garlic cloves with 2 tablespoons of olive oil and the vegetable bouillon powder. Spread on a baking sheet and roast for 25–30 minutes, or until the cauliflower is golden and fork-tender.
If it's not soft enough to blend easily, roast for an extra 5–10 minutes.
Once cool enough to handle, squeeze the roasted garlic out of the skins.
In a blender, combine the roasted cauliflower, peeled garlic, cashews, soy milk, lemon juice, nutritional yeast, and 2 tablespoons of olive oil. Blend until smooth. Season with salt and pepper to taste, then blend again. Add a splash more milk if you'd like a thinner consistency.
Note: Make sure the vegetables are lukewarm (not hot) before blending if using a closed-top blender for safety.
Bring a large pot of salted water to a boil. Cook the fettuccine according to the package instructions. Drain, then return to the pot.
Pour the cauliflower Alfredo sauce over the cooked pasta and stir to coat evenly. Serve with vegan parmesan or cheese of choice, if desired.