Preheat oven to 450. On a large rimmed baking sheet, toss mushrooms with 2 TB olive oil, rosemary and garlic. Season with salt and pepper. Roast until tender and browned, tossing occasionally, about 15 minutes.
Push mushrooms to the sides of the pan. Place tilapia in center and season with salt and lemon pepper. Roast until opaque throughout, 8 - 10 minutes.
Meanwhile, in a small bowl, whisk together remaining oil (2 TB), lemon juice, mustard and parsley, season with salt and pepper.
When tilapia is finished cooking, place on a plate with the mushrooms next to it. Spread the mustard sauce on the top of the tilapia.