Slice sweet potatoes longways into ½" thick slices
Combine oil, spices, salt, and vinegar in a small bowl and whisk until smooth. Pour over sweet potatoes and toss until coated.
Place potato slices on grill and cook for 20-30 minutes, turning frequently until they are tender and pierce easily with a fork.
Transfer into a large bowl and toss with sliced green onions and butter. Using the end of a spoon, break up the potato slices into bite-sized pieces if desired. Serve warm.