In a small pan, heat 2 cups of pineapple juice to simmer. Stir in the cornstarch, a little at a time. The juice will thicken quickly. Continue to stir until it stops thickening. This should take five to six minutes. Remove from heat and set aside.
In a separate pan, mix together the soy sauce, 2 cups of pineapple juice, garlic, ginger, mustard and brown sugar. Simmer and stir until the brown sugar dissolves.
Slowly add the thickened pineapple juice to the larger pan. Stir until it is well blended.
Add the cooked meatballs to the teriyaki sauce. Warm over low to medium heat until the meatballs and sauce are hot. They’re now ready to serve.