In a large skillet over medium heat, cook the bacon until crispy. Remove bacon with a slotted spoon, and drain on paper towels.
Pour out most of the bacon fat from the skillet, leaving about 1 tablespoon.
Add the red onion to the skillet and sauté until translucent, about 3 minutes.
Whisk together the red wine vinegar, olive oil, Dijon mustard, salt, and pepper in a small bowl.
Place the spinach in a large bowl. Immediately pour the warm onion mixture over the spinach and toss gently to slightly wilt the leaves.
Drizzle the dressing over the spinach and toss gently to combine. Top with the cooked bacon pieces.
Serve immediately while warm.