Stir 2 tablespoons melted unsalted butter, 2 tablespoons olive oil, 3/4 teaspoon kosher salt, and 1/2 teaspoon black pepper together in a large bowl until combined. Add 2 pounds cut sweet potatoes (peeled if desired) and 1 tablespoon coarsely chopped fresh rosemary leaves, and toss well to coat.
Transfer the potatoes cut-side up to a 9x13-inch metal baking pan (avoid glass, as it could shatter) or rimmed baking sheet (quarter sheet pan) and arrange in a single layer. Drizzle any remaining butter mixture over the potatoes.
Roast until the sweet potatoes easily release from the pan with tongs and the bottoms are light golden-brown, about 15 minutes. Flip the rounds and roast until the bottoms and tops are deep golden-brown, about 15 minutes more.
Remove the pan from the oven. Carefully pour 1 cup low-sodium vegetable broth into the pan and add 4 peeled and smashed garlic cloves. Return to the oven and roast until the potatoes are very tender and most of the broth is absorbed, about 15 minutes.
