Preheat your oven to 350 degrees F (175 degrees C). Meanwhile, sauté the diced onions and mushrooms in olive oil in a frying pan until they soften. Once done, set them aside.
Line a baking sheet with aluminum foil for easy cleanup. Using a serrated knife, carefully cut the King's Hawaiian rolls in half, separating the top from the bottom. Place the bottom half of the rolls on the prepared baking sheet.
Combine the mayonnaise and Dijon mustard in a small bowl to create a delectable spread. Spread half of this mixture over the cut side of the roll bottoms.
Now comes the meaty goodness! Lay the honey ham slices and smoked turkey slices over the roll bottoms, ensuring the entire layer of bread is covered. Depending on the size of the cuts, you can usually fit eight pieces per layer.
Sprinkle the sautéed onions and mushrooms evenly over the meat layer. Next, add the chopped pickled jalapenos for a spicy kick that will tantalize taste buds.
Lay the provolone cheese slices over the meat and toppings, allowing them to overlap to ensure the gooey, melted cheese covers all the meat.
Spread the remaining mayo and Dijon mixture over the cut side of the roll tops. Carefully place the roll tops over the cheese, cut side down, completing the sandwich layers.
In a small bowl, add the butter, dried parsley, garlic powder, and poppy seeds, melt and stir to combine, creating a flavorful butter mixture. Use a pastry or basting brush to spread this mixture over the sliders' tops generously.
Place the baking sheet with the sliders in the oven and bake for approximately 15 minutes or until the cheese is melted and the rolls are golden and slightly crispy.
Remove the sliders from the oven, and cut them into individual portions with the help of a spatula or sharp knife. Arrange them on a serving platter and watch your guests devour these delectable King's Hawaiian Club Sandwich Sliders.