Step 1Boil the water in a large pot and drop in the sweet potato slices. Boil 3 to 4 minutes, remove, drain and pat dry.
Step 2Combine the mustard, olive oil, butter and rosemary in a bowl and brush both sides of the sweet potato slices. Reserve the rest of the mixture for basting.
Step 3Grill the slices on an oiled rack over medium-high heat (450 to 550 degrees) until fork-tender, about 5 minutes, turning and basting often with the mustard mixture. Remove from the heat and season with salt and pepper to taste. Serve warm.