I ended up making a big pan of mushroom polenta the morning of the Osso Bucco dinner, and it worked out quite well.
I sautéed some crimini and shiitake mushrooms in olive oil and butter, slowly, and got great color and a bit of carmelization (don't stir too much, and don't salt!). I sautéed them in the stainless steel pot in which I planned to cook the polenta. I removed the shrooms and poured in water, deglazing the pan. Brought to a boil, stirred in the polenta and, after a bit, added the mushrooms, some finely minced rosemary, and simmered and stirred for awhile. Just before completion, I threw in about 1/3 cup of sour cream. Poured that into a glass baking dish and chilled.
Before dinner, I cut the polenta into triangles and topped them with good parmesan cheese, placed under the broiler to brown, and served with the Osso Bucco on top. It worked out really well. Thank you all for the suggestions.