Start by heating 1 tablespoon of olive oil in a skillet over medium heat. Add the sliced Vidalia or yellow onion and a pinch of salt. Cook slowly, stirring occasionally, for about 20-25 minutes until the onions become soft and caramelized. Lower the heat if necessary to prevent burning.
Once the onions are caramelized, add another tablespoon of olive oil to the skillet. Add the sliced portobello mushrooms and sauté for about 5-7 minutes, or until they are tender and golden brown. Season with salt and pepper to taste.
In a small bowl, combine the ketchup, mayo (or vegan mayo), and pickle juice. Mix until well combined. Taste and adjust the pickle juice to your preference. If desired, add a pinch of onion powder for extra flavor.
Slice the slider buns in half. On the bottom half of each bun, place a generous spoonful of the caramelized onion mixture, followed by a few pieces of sautéed mushrooms. Top with a spoonful of diced pickles. Drizzle with the fry sauce and place the top half of the bun on each slider.
Arrange the sliders on a serving platter and serve immediately. These sliders are best enjoyed fresh but can also be served with a side of fries or a crisp salad.
