In a blender, combine the shallot, olive oil, champagne vinegar, mustard, salt and pepper. Blend until smooth. Add the tarragon and pulse 8-10 times until tarragon is chopped and incorporated.

In a skillet over medium heat, cook the bacon slices until they are crispy, about 8-10 minutes. Remove the bacon from the skillet and place it on a plate lined with paper towels to drain. Once cooled, crumble the bacon into small pieces.

Add the spinach leaves to a large bowl. Add the mushrooms and bacon. Toss with dressing and serve.

Serve immediately.