Put oil in a large skillet over medium-high heat. When hot, add sweet potatoes and sprinkle with salt and pepper. Cook, stirring only occasionally, until they change color and begin to brown, then stir more frequently until they are tender but not at all mushy.
Meanwhile, heat butter in a small saucepan over medium heat. Add garlic and sage; shake pan occasionally. When butter turns brown, turn off heat.
Use tongs to remove sage and garlic from butter. Serve potatoes drizzled with butter and garnished with a few sage leaves. Garlic can be served alongside, though it will not be super-soft.