1. For Potatoes: Bring a pot of water to a boil. Add potatoes and cook until tender, about 15 minutes. Drain, reserving ½ cup of the starchy boiling water. Using a potato masher or electric beater, slowly blend potatoes until smooth and creamy adding some of the starchy water if needed to get a smooth and creamy texture. Season with salt and pepper to taste. Set aside.
2. For sauce: In small saucepan whisk together 1cup vinegar and Dijon mustard. Add strawberries, salt, and bring to a boil. Reduce heat to a simmer, and simmer until sauce is reduced to ½ cup. Remove from heat and place sauce in an upright blender. Blend until smooth and return to pan. Set aside.
3. For mushrooms: Preheat outdoor grill to medium high. In a medium bowl, whisk together oil, vinegar, Dijon and salt. Brush mushrooms, and bell pepper with the oil and vinegar dressing. Press peppercorns into mushrooms. Grill mushrooms, peppers, and onion with lid down, turning once for 10-15 minutes, until grill marks appear on pepper and onion, and mushrooms are tender when pierced with a fork.
4. When mushrooms are almost done, reheat the potatoes and sauce. Arrange ½ cup mashed potatoes on each of four plates, place one mushroom, cupped side up on top of potatoes, then top with red pepper slices, and an onion slice. Drizzle with balsamic reduction sauce.