Preheat oven to 350℉ and grease a 13x9 baking dish.
Preheat a cooking pan over medium heat and add oil. Add sliced onions and sauté until transparent. Add mushrooms, season with salt and pepper, and cook until both done and browned.
Take the pack of rolls out of the bag and cut them in half, width-wise. Place the bottom half in the baking pan.
Spread gourmet mustard over the bottom buns in a thin and even layer.
Top the buns with 4-6 slices of Swiss cheese, depending on the size of cheese slices, the buns should be covered evenly! You can slightly overlap the slices if needed.
Layer corned beef evenly over the cheese.
Spread cooked mushrooms and onions over the corned beef evenly.
Layer cheese slices over the top, evenly. Cover with the top halves of rolls.
Melt butter and mix it with dried minced garlic and poppy seeds. Brush the top of the rolls with the butter mixture.
Loosely tent the baking dish with the sheet of aluminum foil and bake for 25-30 minutes.
Cut in between each slider to take it out easily.
Notes
Want to use pastrami? Yes, you can definitely use pastrami! Just note that the flavor won’t be quite the same. It will still be delicious, but pastrami tastes different than corned beef.
Storing: Store your leftover sliders in an airtight container in the refrigerator for 2-3 days. If you'd like to store them in the dish they were baked in, go for it! Just make sure to cover it tightly so they don't dry out.
Reheating: To reheat leftover sliders, I recommend doing so in the oven or air fryer. The microwave tends to make the bread soft and soggy. In the oven, reheat the sliders on a parchment paper covered baking sheet at 350℉ just until heated through.
Reheating in the air fryer, I recommend lining the basket with a perforated parchment paper sheet as well and reheat at 360℉ for just a couple of minutes to warm through.