Divide spring mix among 4 serving plates; set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add a single layer of mushrooms and cook, without stirring, for about 5 minutes or until mushrooms become red-brown on one side.
Turn the mushrooms and stir in shallot or onion, red pepper and garlic. Cook for 5 minutes, or until shallot or onions begin to soften. Remove from heat and transfer to a separate bowl.
Stir in vinegar, sugar, salt and pepper and mix until sugar is dissolved. Spoon mushroom mixture on top of greens and scatter fresh chives on top. Serve immediately.