Melt 2 tablespoons butter in a large skillet over medium heat. Add onions, cover and cook over medium heat, stirring, until softened, about 10 minutes. Reduce heat to medium-low. Season with salt and pepper and cook, stirring occasionally, until onions are caramelized and very soft, about 20 minutes. Transfer to a bowl and set aside.
In the same skillet, melt remaining 2 tablespoons butter over medium-high heat. Add the mushrooms and season with salt and pepper. Cook, stirring occasionally, until mushrooms are golden brown and beginning to crisp, 10-12 minutes. Remove from heat and stir in 1 tablespoon Challenge Garlic Parmesan with Herbs Spreadable Butter. Transfer to a bowl and set aside.
Arrange 8 slices of bread on a board and spread each with Challenge Garlic Parmesan with Herbs Spreadable Butter, about ½ tablespoon per slice. Flip half the slices and top each with ½ cup sliced steak. Divide caramelized onions and mushrooms over top. Sprinkle each sandwich with ⅓ cup shredded cheese and top with leftover bread slices, buttered side up.
Preheat a large skillet over medium heat. Carefully transfer one sandwich to the skillet, cover and cook until bread is golden and cheese is starting to melt, 3-5 minutes. Flip, cover and cook another 3-5 minutes, until cheese is fully melted. Repeat with remaining 3 sandwiches. Serve hot.
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