In a skillet add the bacon and cook over medium heat until crisp. Using a slotted spoon transfer the bacon to a paper towel lined plate to drain. Roughly chop and set aside.
Pour off all but 2 tablespoons of fat from the skillet. Heat the remaining fat over medium-high heat, add the shallots and cook for 2 minutes, stirring occasionally. Whisk in the vinegar, desired amount of mustard, and salt and pepper, to taste. Scrape the brown bits from the bottom of the skillet, bring to a simmer, then remove the skillet from the heat.
For the salad, in a large salad bowl, toss together the spinach, mushrooms, bacon and onion. Pour the dressing over the salad and toss to combine. Serve warm.