Heat the olive oil in a large skillet over medium heat.  Sauté the onion and Portobello mushroom for 3 minutes.  Add the rice and stir to coat it with oil.  Add the wine and stir until the wine comes to a boil.  Reduce heat to a simmer, stirring constantly, until the wine is reduced by half.  Add the heated stock, ½ cup at a time, stirring constantly until almost all the liquid is absorbed before adding the next ½ cup (about 25 minutes of stirring). Stir in the Parmesan and butter. Salt to taste and serve immediately.