In a large skillet over medium-high heat, add the oil, and when it is hot, add enough mushrooms just to cover the bottom of the pan (ultimately, you'll probably cook the mushrooms in 2 batches). Cook the mushrooms without stirring until browned on 1 side and then turn the mushrooms. Continue cooking, stirring occasionally, until the mushrooms are well browned on both sides, about 5 minutes. Remove to a plate and repeat with the remaining mushrooms, adding a little more oil to the pan if necessary.
Scrape the mushrooms back into the pan. Over medium heat, add the shallot and saute until tender, about 5 minutes.
Meanwhile, put the baguette slices on a broiler pan, brush both sides with some olive oil and season with salt and pepper. Broil until toasted, 2 to 3 minutes.
Once the shallot is tender, add the tarragon and vinegar and cook until the mushrooms absorb the vinegar. Sprinkle with the parsley and season with salt. Add the butter pieces and swirl the pan until the butter melts and forms a sauce. Serve immediately over the baguette slices.