Chop up the vegetables (and meat if you want it)
You want it all sliced thin so they'll cook quickly
Put a tablespoon of cornflour in a mug. Add the pineapple juice and stir. You do NOT want big lumps. Cornflour and juice is a "Newtonian" liquid... which means it's half liquid half solid and if you BASH it suddenly it becomes a solid. This is bad news as far as lumps are concerned.
Add soy sauce, you'll need about a dessert spoon. The sauce won't look all that great but it will taste terrific.
Fry the veggies and pineapple together on a hot heat. Pour the sauce over the top. Serve immediately.