Preheat the oven to 400 degrees F. Soak the sprouts for a few minutes in a bowl of water to remove any dirt. Trim the ends off the sprouts and remove any leaves that don't look good.
Slice the Brussels sprouts in half and place them on a parchment paper lined baking sheet. Clean the mushrooms and slice them in half and add to the baking sheet as well. Drizzle with 2 Tablespoons olive oil.
Bake at 400 degrees F for about 35-40 minutes.
In a small bowl, whisk together the dressing ingredients.
Put the roasted mushrooms and Brussels sprouts in a serving bowl and drizzle with the dressing. Toss to coat. Season with salt and pepper if desired.
Notes
Store leftovers in the fridge. They should stay fresh for about 3 days, but they are really best when they are first made.