Melt the butter in a large skillet over medium heat. Saute the ham, onions, green peppers, and pineapple chunks in the butter for about 5 minutes.
In a separate medium bowl, combine the pineapple juice, vinegar, brown sugar, mustard, and cornstarch. Stir this together well and pour over the ham mixture in the skillet. Stir well and allow to heat through and thicken, about 5 - 10 minutes. Serve over rice.