Heat a skillet on medium-low heat and add 1/2 tablespoon olive oil. Add sliced onions with a pinch of salt and let caramelize, stirring occasionally. Cook for about 8-10 minutes, then turn off heat and set aside.
While onions are caramelizing, brush mushrooms with remaining olive oil and sprinkle both sides with pepper, paprika, onion and chili powder. Heat a skillet ( or a grill) on medium heat and cook until mushrooms are juicy and tender – about 5 minutes on each side. With 1-2 minutes remaining, add a slice of gouda on top to melt.
Assemble sliders by placing the mushroom on the bun, then top with onions, avocado, and a hefty drizzle of bbq sauce.
Note: if you can’t find slider buns in your local store, just buy dinner rolls. I did this for years before grocery stores actually started selling slider buns!