Heat the oil in a 6-quart pot over medium heat. Add the onion and cook, stirring often, until tender and golden-brown, 5 to 8 minutes. Add the mushrooms and garlic and increase the heat to medium high. Cook, stirring often, until the mushrooms are tender and browned, 10 to 14 minutes.
Add the butter. Once it melts, add the rice and stir until coated. Add the wine and cook, stirring, until it evaporates, about 2 minutes. Stir in 4-3/4 cups of the broth, the lemons, marjoram, and 1/4 tsp. salt and simmer over medium-low heat, stirring occasionally, until the rice is almost tender, about 20 minutes. Add the chicken, 2 Tbs. of the pan juice, and 1/4 cup of the remaining broth. Cook, stirring, until the risotto is creamy and tender, adding the remaining 1/4 cup broth and more pan juice to taste, about 5 minutes. Stir in the Parmigiano and season to taste with salt and pepper. Spoon the risotto into warm shallow bowls, sprinkle with more Parmigiano and the parsley, and serve.