Rinse portobellos and cut them in six pieces. In a skillet combine olive oil and balsamic vinegar. Saute mushrooms in this mixture for a few minutes. Season with salt and pepper and set aside.
In a large pan saute onion in oil and butter until tender but not colored. Season.
Add rice and stir for 2 minutes. Gradually add hot stock, not boiling, stirring until liquid is absorbed. Once liquid is absorbed add more stock and wine and continue stirring. Repeat this until rice is cooked. Process will take 18 to 20 minutes. Once rice is cooked add cream, parmesan cheese and the portobellos with the vinegar and olive oil mixture. Mix well. Correct seasoning.
Serve hot, immediately.