When the polenta is done, immediately whisk in the cooked onions and cauliflower, the grated cheese or nutritional yeast. Spread the hot polenta cauliflower mixture into the well-oiled 10-inch springform pan, cover with plastic wrap directly on top of the polenta, and refrigerate until set, about an hour. (At this point you can refrigerate the polenta in its dish for a day or so.)
Preheat oven to 425°F.
Brush the top of the torta with olive oil and bake 40 to 45 minutes, until it takes on a nice golden color. (If the uncooked torta is cold, you will need to increase the baking time 5 to 10 minutes or finish under the broiler.)
Cool 10 to 15 minutes before cutting, to let the hot torta set.
Wash the asparagus and snap off the tough ends of the stalks. Setting aside 12 pieces for the top, cut the rest in 2-inch lengths. Place on a tea towel to dry away most of the moisture, then toss both the lengths and pieces with olive oil, salt, and about 20 grinds of black pepper. Spread on a baking sheet. Roast 15 to 25 minutes, until the tips begin to blister and brown, shaking the pan halfway through. Roasting time will depend on the thickness of the stalks.