1. Preheat grill to high heat.
2. Brush mushrooms with olive oil and season with salt, pepper, and garlic powder.
3. Grill for 10 minutes per side. Allow to rest for five minutes, then cut into thin strips.
4. To make the dressing: Combine shallot, mustard, vinegar, lemon juice, honey, parsley, salt, and pepper in a bowl. Whisk in oil in a thin stream until emulsified.
5. Assemble salad and toss with dressing.