Preparation Place mushrooms in a steamer basket and set over a pot filled with 1" gently simmering water (do not allow water to touch mushrooms). Cover and steam mushrooms until tender, about 5 minutes. Transfer mushrooms to a baking sheet and let cool slightly. Meanwhile, whisk oil, parsley, vinegar, Dijon mustard, shallot, and garlic in a medium bowl. Season with salt, pepper, and lemon juice. Add mushrooms; toss to coat evenly. Let marinate for 30 minutes. Season to taste with salt, pepper, and more lemon juice, if desired.
Preparation Place mushrooms in a steamer basket and set over a pot filled with 1" gently simmering water (do not allow water to touch mushrooms). Cover and steam mushrooms until tender, about 5 minutes. Transfer mushrooms to a baking sheet and let cool slightly. Meanwhile, whisk oil, parsley, vinegar, Dijon mustard, shallot, and garlic in a medium bowl. Season with salt, pepper, and lemon juice. Add mushrooms; toss to coat evenly. Let marinate for 30 minutes. Season to taste with salt, pepper, and more lemon juice, if desired.