I came up with this one Thanksgiving because I was short on oven space. Everyone loved it and I have made it for every Thanksgiving and holiday meal ever since. Be careful not to overcook marshmallows, they can become liquified.
To elevate this dish, consider using fresh sweet potatoes; peel, boil until tender-crisp, then slice or mash slightly before adding to the slow cooker for a superior texture and flavor. For an extra layer of complexity, a pinch of cinnamon, nutmeg, or allspice can be added with the brown sugar. A splash of orange juice or even a hint of bourbon can also complement the sweet potatoes beautifully. Crucially, monitor the marshmallows closely in the final stage. They can go from perfectly puffy to completely melted and absorbed very quickly. If you prefer a slightly browned, toasted marshmallow topping, carefully transfer the sweet potato mixture to an oven-safe dish after the slow cooking, top with marshmallows, and broil for 1-2 minutes until golden.