At least 1 hour before grilling, soak plank in enough water to cover. Weight down plank to keep it submerged during soaking. Drain plank.
In a medium skillet, cook mushrooms, onion, and garlic in hot oil over medium-high heat until tender. Remove from heat. Let stand about 5 minutes to cool slightly. Stir in spinach, cheese, and prosciutto. Set aside.
Remove bone from chicken breasts, leaving skin attached. Spoon mushroom mixture between the skin and flesh of each breast half. Brush chicken with olive oil and sprinkle with pepper.
For a charcoal grill, place plank on the grill rack directly over medium coals; heat about 5 minutes or until plank begins to crackle and smoke. Place chicken, skin side up, on plank. Cover; grill chicken about 20 minutes or until no longer pink (170 degrees F). (For a gas grill, preheat grill. Reduce heat to medium. Grill chicken as above.)
Remove chicken from grill. Let chicken stand for 5 minutes. Slice chicken diagonally and serve over Fresh Herbed Pasta. Makes 4 servings.
Fresh Herbed Pasta
Cook linguine according to package directions; drain. Meanwhile, in a large skillet, cook shallot and garlic in hot olive oil over medium heat until tender. Add chicken broth and dry white wine. Bring to boiling; reduce heat. Simmer, uncovered, until mixture is reduced to 1/3 cup. Remove from heat. Add drained pasta, red sweet peppers, basil and/or parsley. Toss until combined. Sprinkle with Parmesan cheese. Sprinkle with freshly ground black pepper to taste.