In a bowl combine spicy brown mustard, peppercorn dressing, asiago cheese, garlic and pepper seasoning, and black pepper until blended. Hold refrigerated for service.
In a bowl combine extra virgin olive oil, Worcestershire sauce, soy sauce, Grill Mates® Montreal Steak® Seasoning, and mustard seed. Whisk until blended.
Divide marinade in half and separately marinate steak and mushrooms. Toss until well coated. Cover and marinate at least 1 hour, up to overnight.
Set grill to medium high heat. Remove steak and mushrooms from marinade. Grill steak until desired doneness and mushrooms until soft and slightly charred.
Allow to rest and slice steak and mushrooms 1/4" thick. Hold warm for service or chill and refrigerate until needed.
Set a panini press to medium heat. For each serving, spread a tablespoons of mustard sauce on inside of 2 slices of bread.
Spread 1/4 cup asiago cheese over one slice of bread. Top with 4 ounces sliced steak, 1/4 cup bell pepper, and 1 caps worth of mushrooms on one slice of bread. Finish with 1/4 cup asiago cheese and second slice of bread.
Butter outside of sandwich with 2 tablespoons of butter. Place in panini press and grill. Remove from press and cut in half. Serve Hot.
Add 1/4 cup of French's® Crispy Fried Onions to each sandwich for an added boost of texture.
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