1. In a skillet, cook bacon until crisp, about 10 minutes. Drain on paper towels and let cool, then crumble.
2. Cut mushrooms into strips and toss gently with olive oil, salt, pepper, thyme and rosemary. Warm a large skillet over medium-high heat. Cook mushrooms, stirring often, until tender and water has drained out, about 8 minutes. Remove from skillet to a bowl and let cool.
3. Whisk together vinegar, mustard, salt and pepper. Slowly drizzle in olive oil, whisking constantly. In a large salad bowl, toss spinach, onion and mushrooms gently with dressing. Sprinkle bacon on top and serve.