Fry the bacon until crisp, transfer to a paper-towel lined plate and reserve 3 tablespoons of the fat. Crumble or cut the bacon into small pieces and set aside.
Place the mushrooms in a bowl and set aside.
Add the vegetable oil to the bacon drippings and heat over medium heat. Add the balsamic vinegar, Dijon-style mustard and sugar and whisk until the sugar dissolves and the mixture is well combined. Add the scallions and cook for 1 minute to soften. Pour the entire mixture over the mushrooms, stir to coat and season to taste with salt and pepper.
Place the spinach in a large salad bowl, add the mushroom-dressing mixture and toss. Taste and adjust the seasoning as needed.