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Preheat oven broiler to high and set top oven rack to the second highest setting.
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Add ground beef, mustard, garlic, and onion to a medium bowl. Season with salt and black pepper, to taste, and mix with a fork or your fingers until all ingredients are thoroughly combined.
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Divide beef mixture into 8 portions and shape each into small patties. Set patties on a plate and gently press each with your thumb in the center to keep them from puffing up while cooking. Set aside.
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Heat olive oil over medium-high heat in a large, oven-safe skillet. Add mushrooms and cook, stirring occasionally, until mushrooms are nicely browned, approximately 5-6 minutes. Remove from heat and transfer mushrooms to a small bowl and set aside.
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Add remaining tablespoon of olive oil to pan and swirl to coat the bottom. Add slider patties to the skillet and cook until brown on each side, approximately 2-3 minutes per side.
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Remove from heat and top each slider with sautéed mushrooms and crumbled blue cheese. Place skillet in oven under broiler for 2-3 minutes, or until blue cheese starts to melt.
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Remove skillet from oven and top each slider with a piece of bacon and serve immediately on a bed of lettuce and with a side of steamed broccoli, if desired. Enjoy!