- In a large skillet over medium heat, cook bacon until crisp, about 8-10 minutes.
- Transfer to paper towels to drain, reserving a thin coating of drippings in skillet.
- Crumble bacon when cool enough to handle. Set aside.
- Add lemon juice, brown sugar and mustard to drippings in skillet and cook over low heat, stirring, for 1 minute.
- In a salad bowl, toss spinach, mushrooms and red onion with dressing.
- Serve sprinkled with crumbled bacon.
NOTE: if you don't want to use bacon drippings, combine the lemon juice, brown sugar and mustard with vegetable oil and season with salt and pepper. Adjust seasonings to taste.