Preheat your oven to 190° C (375° F), get out a 7x11 baking pan.
In a large non stick pan, melt the butter, when hot, add the onion and cook until tender but not browned.
Add the pork and beans, ketchup, and the 1 tablespoon of mustard. Bring to a boil.
Pour into the baking pan.
Cut the luncheon meat into 6 slices, cut slices in half lengthwise to give you 12 strips. Arrange the strips on top of the beans.
Arrange the pineapple slices down the center of the pan.
In a bowl, mix together the brown sugar, 1 tablespoon of mustard, and the pineapple juice. Spread over the meat and pineapple.
Bake for 15-20 minutes or until heated through.
Serve and enjoy.