In large saucepan, cook orzo in salted water. Drain, reserving 1 1/4 cups water from cooked pasta.
In large, deep nonstick skillet, heat oil over medium-high heat. Add asparagus and cook, stirring occasionally, for 2 minutes. Add garlic and thyme, and cook 30 seconds more. Stir in peas, spinach, lemon peel, and reserved pasta cooking water. Increase heat to high, and bring to a boil. Stir in orzo, butter, and lemon juice. Transfer to serving bowl, and sprinkle with feta cheese and mint.