Season chicken livers with salt and pepper. Heat olive oil over high heat and sear chicken livers, browning both sides. Remove from skillet while still rare with a slotted spoon and reserve. Add chopped onion and garlic to the same skillet and cook over medium heat, stirring to prevent browning. Stir in rice and Portobello mushrooms and cook for 3 minutes, approximately. Add rice and stir with a wooden spoon until coated.
Reserve 2 tablespoons of the champagne and add the remainder to the skillet. Stir constantly over medium heat until champagne has been absorbed. Add ¾ cup hot chicken stock and stir until it is absorbed. Continue adding stock, ¾ cup each time, letting each addition be absorbed before adding the next one. This process takes from 18 to 22 minutes. Rice should be al dente.
Five minutes before rice is ready stir in snow peas and chicken livers. Add butter and the reserved 2 tablespoons of champagne.
Stir and add half of parmesan cheese. Correct seasoning.
Serve immediately. Serve with remaining parmesan cheese.