Heat 1/2 tablespoon each of the butter and oil in a large, heavy-based saucepan over medium heat. When the butter starts to foam, add the onions; cook until softened, 5 to 8 minutes. Add the garlic; cook until softened and fragrant, 2 to 3 minutes.
Add the barley and stir until well-coated in the oil. Pour in the wine and simmer, stirring, until the liquid has been absorbed. Add all of the stock and simmer gently, stirring from time to time, until the liquid has been absorbed, about 40 minutes. (Alternatively, drain off any excess water as soon as the barley is plump and tender and just passed al dente.)
About 10 minutes before serving, in a separate pan, add the remaining 1/2 tablespoon butter and olive oil. When the butter starts to foam, add the mushrooms and cook until lightly golden, 3 to 4 minutes. Add the mushrooms to the risotto.
Remove the risotto from the heat and stir through the parsley, Parmesan, mascarpone, and salt and pepper to taste.
Serve in pre-warmed bowls, garnished with parsley. Eat immediately.