Combine Diana Sauce, lemon juice, garlic, green onions and rosemary in bowl. Remove 1/4 cup (50 mL) for basting.
Cut mushrooms into large chunks, keeping oyster mushrooms whole. Toss with onion, and salt and pepper to taste. Pan-fry in butter in nonstick skillet over medium heat for about 12 minutes or until slightly caramelized and liquid has evaporated. Stir in Diana Sauce mixture; heat through.
Meanwhile, season medallions with salt and pepper to taste. Grill over medium-high heat for 5 to 6 minutes per side for medium-rare, brushing with reserved sauce occasionally. Let stand for 5 minutes.
Serve each topped with pan-fried mushrooms.
Per Serving: 305 Calories, 29 g protein, 10 g fat, 25 g carbohydrate. Excellent source of iron (25%DV) and zinc (69%DV), source of vitamin D (10%DV), 14%DV sodium.