In medium sauce pan, melt butter and add shallots and pepper flakes over medium heat.
Sauté three minutes then add sherry. Cook to evaporate the sherry, about two minutes.
Add apple juice and bring to a boil then reduce to a fast simmer and cook down to a syrup, about ten minutes.
Add cola and bring to a boil then reduce to a fast simmer and cook down to a syrup, about ten minutes.
Add brown sugar, mustard and ketchup and simmer until thick and sticky, about five minutes. (About the consistency of a thick maple syrup).
Turn the burner off and leave sauce in the pan.
Heat a grill pan over medium heat and once hot, lay out half of the kielbasa slices and cook for five minutes. Turn each slice and cook for five more minutes. Add this batch to the sauce but leave the fat behind.
Pour the fat into a small bowl and wipe the pan clean.
Repeat for the second half of the kielbasa, again adding to the sauce and pouring the fat into the small bowl.
Turn the burner back on under the sauce at a low heat to stay warm while you grill the pineapple.
Wipe the pan clean and heat back over medium.
Pour two tablespoons of the reserved fat onto the pan and once hot, place all of the pineapple pieces onto the hot fat.
Grill the pineapple for five minutes per side and add to the pan with the kielbasa.
Serve hot as an appetizer.
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