In a large pot or Dutch oven, melt the butter and olive oil over medium-high heat. Add the shallots and cook until soft and translucent, 3 to 4 minutes. Add the garlic and cook until the garlic is soft and fragrant, 1 to 2 minutes more. Add the sweet potatoes, rosemary, vegetable stock, water, salt and pepper. Bring the mixture to a boil, then reduce the heat, partially cover the pot, and simmer until the sweet potatoes are fork-tender, about 20 minutes.