Preheat oven broiler to high and set top oven rack to the second highest setting.

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. 

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. 

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.

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. 

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. 

Remove skillet from oven and top each slider with a piece of bacon and serve immediately on a bed of lettuce.

If using broccoli as side, steam in lightly salted water and serve. Enjoy!