3 cups Diced onion and sliced white mushrooms (each)
3/4 cup Mayonnaise
1/2 cup Butter
1 loaf Italian bread, sliced into 12 ½-in (1 cm) thick pieces
3 Whole tomatoes, sliced into 12 pieces
12 slices Gruyère cheese
In a clean resealable plastic bag, add steak and 10 tbsp of seasoning and 3 tbsp (45 mL) olive oil.
In a large sauté pan on medium-high heat, add remaining oil, onions and mushrooms; sauté for 2 minutes. Chill and reserve.
In a medium bowl, combine mayonnaise and the remaining seasoning; mix well.
Place steak onto a hot grill and grill until desired doneness. Let rest for 5 minutes.
Butter one side of 6 bread slices and on the other side spread with prepared mayonnaise. Divide onions and mushrooms between slices of bread and place on the mayonnaise topped bread slices. Add 2 slices of tomatoes.
Sliced steak into thin slices and add on top of tomatoes. Top with a slice of gruyere cheese and then the remaining bread slices, making sure buttered side is facing out.
Place into a hot Panini press and cook until golden brown.