Heat olive oil in a large skillet over medium-low heat. Cook onion in hot oil until softened, about 3 minutes. Add bell pepper and portobello mushrooms to the skillet, season with kosher salt and pepper, and continue cooking another 10 minutes.
Stir Arborio rice into the mushroom mixture. Pour white wine into the skillet; cook and stir until the wine is warmed. Add chicken stock 1 1/2 cups at a time, stirring continually to allow each addition to absorb into the rice before adding the next, until all stock is integrated, 20 to 30 minutes. Stir heavy cream into the rice mixture; cook about 1 minute. Remove the skillet from heat and stir Parmesan cheese into the risotto. Let sit 5 to 10 minutes before serving.