Drop onions into a saucepan of boiling water; return to a boil. Drain; rinse under cold water. Drain and peel.
Heat oil in a nonstick skillet over medium heat. Add onions; sauté 5 minutes. Add peas and carrot pieces; sauté 5 minutes or until crisp-tender. Remove from heat; stir in mushrooms, parsley, and mint. Spoon vegetable mixture into a shallow dish or glass bowl; set aside.
Combine the vinegar and the next 4 ingredients (vinegar through salt) in a small saucepan; bring to a boil. Pour over vegetables; cover and marinate in refrigerator at least 2 hours. Serve with a slotted spoon.