Heat Blackstone griddle to medium (375–400°F). Lightly oil the surface with about a Tablespoon of cooking oil.
Whisk all honey mustard sauce ingredients in a bowl. Set aside.
Cook bacon on griddle until crispy. Remove and push to a cooler side of the griddle.
In bacon fat, sauté mushrooms and onion until tender. Remove and push to a cooler side of the griddle with the bacon.
Add the chicken breasts to the grill and season chicken with salt and pepper.
Grill 5–6 minutes on one side untouched until the chicken is browned and a nice crust is formed. Flip the chicken and cook the other side for 5-6 minutes until the other side of the chicken is browned as well.
Lower heat to medium-low. Spread honey mustard sauce over chicken.
Top with mushrooms, bacon, and shredded cheese.Cover with grill dome or tent loosely with foil.
Cook 3–5 minutes until cheese melts and chicken reaches 165°F.
Remove from heat, serve warm and enjoy!