For the sliders: In a large bowl, combine the beef with some garlic powder, paprika, thyme, Worcestershire, salt and pepper and gently mix. Measure out the beef mixture in a 1/3-cup measure to ensure equal slider size. Roll the measured beef mixture into balls.
Heat a skillet over medium heat and smash the balls in the skillet to create slider patties. Cook to medium, then choose from the following topping combinations.
For the shroom swissalaka sliders: Add slices of Swiss cheese to the sliders in the skillet and let the cheese melt. Top with Feltner Sauce, Grilled Onions and Dad's Mushrooms and serve.
For the brobecue sliders: Add slices of Cheddar to the sliders in the skillet and let the cheese melt. Top with Feltner Sauce, Grilled Onions, BBQ sauce and bacon and serve.
For the guac-this-way sliders: Add slices of Cheddar and pepper Jack cheese to the sliders in the skillet and let the cheese melt. Top with Feltner Sauce, guacamole, lettuce, tomato and tortilla chips and serve.
Combine the mayonnaise, ketchup and sriracha in a bowl and mix together.
Heat the oil in a large skillet over medium heat. Add the onions, salt, pepper and garlic powder and stir. Cook, stirring occasionally, until the onions are softened and start to caramelize, 12 to 15 minutes.
Melt the butter in a slow cooker on high. When the butter is melted, add the soup and season with salt and pepper. Stir the soup, then add the mushrooms. Coat the mushrooms with the soup, add the lid to the slow cooker and cook on high, stirring occasionally, for 5 hours.
Remove the lid and continue to cook on high, uncovered, for 2 more hours to cook down the sauce and allow it to thicken. Continue stirring occasionally. A small amount of liquid should be present upon completion. Reduce the temperature to low or warm for serving during the party. The sauce will continue to thicken during this time.