Heat large skillet ove rmedium heat. Cook the bacon until crispy, 5-7 minutes. Transfer to a paper towel on a plate and reserve.
Add the onion and mushrooms to the hot pan and cook until the onions begin to brown, about 3 minutes. Season the shrimp with salt and pepper and add to the hot pan. Cook until shrimp are pink and firm, about 4 minutes. Stir the mustard and vinegar into the pan; season with salt and pepper. If the pan looks dry, add a splash of olive oil. To serve, top spinach with hot shrimp mixture and some of the liquid in the pan. Sprinkle with the bacon.