Warm the broth in a saucepan over low heat; keep it just below a simmer.
Soak porcini in the boiling water for 20 minutes. Lift out, chop, and strain soaking liquid to remove grit; reserve.
Heat olive oil and 1 tbsp butter in a wide pot over medium heat. Soften onion for 4–5 minutes, then stir in garlic.
Add rice and toast, stirring, until the edges look translucent, about 2 minutes.
Pour in the wine and cook, stirring, until mostly absorbed.
Stir in chopped porcini. Add a ladle of hot broth plus a splash of the strained soaking liquid. Stir until nearly absorbed.
Continue adding hot broth a ladle at a time, stirring often, until rice is creamy and al dente, 18–20 minutes.
Off heat, stir in remaining butter, Parmesan, and mascarpone. Season with salt and pepper; loosen with broth if needed.
Let rest 2 minutes, then sprinkle with parsley and serve immediately.