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.