In a medium-size bowl, combine pineapple juice, tamari, vinegar, ginger, and onions.
Add garlic by pushing it through a garlic press into the bowl.
Slice the pork against the grain into paper-thin pieces, add to bowl and let marinate for 4 hours.
Cut pineapple in half lengthwise, leaving green top intact.
Reserve one half for another use.
With a sharp knife, loosen flesh from the other half, scoop out with a spoon, and cut into 1/2-inch strips.
Set the pineapple and shell aside.
Drain marinade from pork, reserving 1/2 cup.
Heat a wok or large no-stick skillet over medium-high heat and add 3 tablespoons of marinade.
Stir-fry pork until just brown.
Don't overcook.
Remove from pan and add the rest of the reserved marinade plus cornstarch and stir constantly, until thick, about 20 seconds.
Quickly add green peppers, red peppers, pineapple strips, and pork.
Toss and remove from heat.