Place the top oven rack on the second highest setting, then preheat the oven broiler to high.
To a medium bowl, add ground beef, mustard, garlic, and onion. Season with salt and pepper to taste. Mix with a fork (or your fingers) until ingredients are thoroughly combined.
Divide the mixture into 8 equal portions and shape into small patties. Put the patties on a plate, and then gently press the center of each patty with your thumb. (This will keep them from puffing up while they cook.) Set patties aside.
In a large, oven-safe skillet, heat olive oil over medium-high heat. Add mushrooms. Cook, stirring occasionally, until the mushrooms have browned (about 5-6 minutes). Remove from heat, transfer them to a small bowl, and then set aside.
Add the remaining tablespoon of olive oil to the pan. Swirl to coat the bottom of the pan with the oil. Add patties to the skillet and cook until brown on each side (about 2-3 minutes per side).
Remove patties from heat. Top each one with sautéed mushrooms and blue cheese. Place them in a skillet under the broiler for 2-3 minutes, or until the cheese begins to melt.
Remove skillet from oven. Top each patty with a piece of bacon. Serve immediately on a bed of lettuce.