Toppings (optional): thinly sliced red onion, crumbled feta
In Round or Multipurpose Steamer, whisk together marinade mix, juice, and oil. Pour half into a Prep Bowl; cover and refrigerate for later. Add mushrooms to steamer; toss to coat. Cover; marinate 1 hour or overnight.
Preheat grill to medium-high. Lightly brush grill with oil to prevent sticking.