Cook the bacon in a pan and set aside reserving 2 tablespoons of bacon grease.
Caramelize the onions in the remaining bacon grease and set aside, then caramelize the mushrooms in the remaining bacon grease and set aside.
Add the vinegar to the pan and deglaze it.
Add the reserved bacon grease, sugar, mustard, garlic and season with salt and pepper and remove from heat.
Assemble salad, toss with dressing, and serve immediately.
Recipe by The Taste Place at https://www.thetasteplace.com/closet-cookings-spinach-salad-with-bacon-carmelized-onions-and-mushrooms-in-a-warm-bacon-pan-dressing/