Heat olive oil in skillet. Add onions and mushrooms; cook until translucent. Add rice and lemon zest and cook for one minute while stirring. Add lemon juice and cook on medium until nearly evaporated; continue to add stock one cup at a time, still stirring, until liquid cooks off. When all liquid is incorporated and rice is tender (approximately 30 minutes), add peas, Parmesan cheese, salt, and pepper. Plate with halved egg on top. Garnish with chives (optional).