Steam cauliflower and garlic until tender (8–10 minutes).
Drain and transfer to a blender with broth, milk, olive oil, nutritional yeast, and seasonings. Blend until smooth and creamy. Adjust consistency with extra broth if needed.
Vegetables
Preheat oven to 425°F.
Toss broccoli, zucchini, and mushrooms with a drizzle of olive oil. Roast on a sheet pan for 20 minutes, until just tender.
After roasting lower the oven temperature to 375°F.
Wilt spinach in a skillet, then drain and squeeze out all excess liquid, using paper towels or cheese cloth.
Assemble & Bake
Lightly oil a 9×13″ baking dish.
Layer roasted veggies and spinach evenly in the dish.
Pour cauliflower Alfredo sauce over the top, spreading to coat.
Sprinkle breadcrumbs and nutritional yeast evenly.
Bake uncovered at 375°F for 20 minutes, until golden and bubbling.