How to Make It
In a large bowl, mix 1/4 cup of the oil with the lemon juice, mustard, shallot and garlic. Season with salt and pepper.
In a large skillet, heat the remaining 1 tablespoon of oil. Add the mushrooms, season with salt and pepper and cook over high heat, stirring, until any exuded liquid evaporates and the mushrooms are browned, about 8 minutes; keep warm.
Add the arugula and fennel to the dressing in the bowl and toss well. Scatter the mushrooms over the salad, top with the goat cheese and serve at once.