Mushroom Farrotto

Name: Mushroom Farrotto
Ingredients: ⅓ cup extra-virgin olive oil; 8 oz. mushrooms (such as crimini or white button), sliced; 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more; Freshly ground black pepper; 1 medium onion, coarsely chopped; 1 Tbsp. finely chopped rosemary; Pinch of crushed red pepper flakes (optional); 1 medium sweet potato, peeled, cut into ½ pieces; 2 cups pearled or semi-pearled farro, rinsed; 4 cups low-sodium vegetable broth; 2 Tbsp. unsalted butter (optional); Finely grated Parmesan (for serving; optional)