1. Melt 4 tablespoons butter in Dutch oven over medium heat. Add onion and 3/4 teaspoon salt and cook until softened, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Add rice and cook, stirring, until grains are translucent around edges, about 3 minutes.
2. Add wine and cook, stirring constantly, until fully absorbed, about 3 minutes. Stir in broth, pot roast, 1 cup water, and mushroom and onion garnish and bring to simmer. Reduce heat to medium-low, cover, and cook until most liquid has been absorbed and rice is al dente, 13 to 16 minutes, stirring twice during cooking.
3. Add remaining 3/4 cup water and stir until creamy, about 3 minutes. Off heat, stir in Parmesan and remaining 2 tablespoons butter. Cover and let sit for 5 minutes. Stir in parsley and season with salt and pepper to taste. Serve, passing extra Parmesan separately.
Chef's Notes:
For looser risotto, stir in an extra 1/4 cup of water before serving.