Cost per Serving:
1. Melt 1 Tbsp. butter in a skillet with oil over medium heat. Sauté onion until softened, 5 to 6 minutes. Add rice; stir until coated with fat and beginning to toast, about 1 minute. Stir in wine and continue to cook until most of it evaporates, about 3 minutes. Transfer rice mixture to slow cooker.
2. Stir broth, squash and 1 tsp. salt into cooker. Cover and cook on high until almost all liquid has been absorbed and rice is tender but not mushy, 2 to 2 1/2 hours.
3. Warm a nonstick skillet over medium-high heat. Cook prosciutto until browned, about 2 minutes, turning once. Transfer to a plate lined with paper towels. Tear prosciutto into pieces.
4. Stir Parmesan and remaining 1 Tbsp. butter into rice mixture. Divide among 4 bowls; sprinkle with prosciutto. Season with salt and pepper, if desired.