1. At least 1 hour before grilling, soak plank in enough water to cover. Place a weight on plank so it stays submerged during soaking.

2. If present, remove stems from four of the mushroom caps; chop stems. Set aside. Chop remaining mushroom cap; set aside. Scrape and discard gills from the remaining four mushroom caps; set aside.

3. For filling, in a large skillet heat oil over medium heat; add chopped mushroom and stems, onion, and garlic. Cook and stir about 5 minutes or until vegetables are tender. Stir in spinach; cook and stir about 2 minutes or until wilted. Remove skillet from heat. Stir in bread crumbs, cheese, pecans, sage, and mustard until combined. Set aside.

4. For a charcoal or gas grill, place plank on the rack of an uncovered grill directly over medium heat for 3 to 5 minutes or until plank begins to crackle and smoke. Meanwhile, grill mushroom caps, stemmed sides up, on grill rack about 3 minutes or until grill marks appear. Place mushroom caps on plank, stemmed sides up. Spoon filling into caps. Cover and grill about 15 minutes or until mushrooms are brown and filling is heated through.

5. Transfer plank with mushrooms to a serving platter.

Tips

To clean mushrooms, use a damp paper towel to wipe each mushroom. If you wish, you can lightly rinse with cool water and pat dry, but do not soak them. Mushrooms absorb water like a sponge, and waterlogged mushrooms won't brown well.