Heat the lobster stock in a small saucepan; keep warm over low heat. Set a medium heavy pot over medium heat, add the rice and 1 cup of the warm stock, and bring to a boil, then reduce the heat to a gentle simmer and cook, stirring, until the rice has absorbed most of the stock. Add another cup of stock and cook, stirring, until most of the stock has been absorbed. Continue to cook, stirring and adding stock each time the previous addition has been aborbed, until the rice is tender; this will take about 20 minutes.

Meanwhile, heat the olive oil in a large sauté pan over high heat. Add the mushrooms and sauté for 5 to 8 minutes, or until browned and softened. Stir the mushrooms into the rice, season with salt and pepper to taste, and serve.