Peel and cut the sweet potatoes. Cut the pointy ends flat so that they stand up straight.
Pre heat the oven to 400F / 200c.
Heat a cast iron skillet on a high heat. Once hot add the butter and oil. When the butter has melted and is starting to foam, Place the sweet potatoes in the skillet, so that they are standing tall and season with a little salt and pepper.
Once the bottoms have caramelized, about 5 minutes, flip them over so that the cooked side is facing up. Turn the heat down to medium
Add the garlic and fresh herbs to the skillet and cook til fragrant. Toss the potatoes in the seasoned oil and butter to coat.
Place the sweet potatoes so that the cooked, caramelized side is facing up. and add the stock to the skillet.
Transfer the skillet to the oven and cook for 25-30 minutes until the sweet potatoes are fork tender.