Cook orzo according to package directions. Drain and rinse under cold water to stop the cooking. Toss with 1 tablespoon olive oil. Reserve.
Heat a large heavy bottomed, wide-surfaced sauté pan, until very hot.
Reduce heat to medium-high. Add vegetable oil and sliced onions. Cook onions on high for about 10 minutes, adding up to 1 tablespoon more oil if pan becomes dry. Reduce heat to medium-low, add garlic and continue to cook for another 20 minutes until golden.
In a large bowl, combine reserved orzo with green onions, feta cheese, baby spinach, mint, olive oil, lemon zest and lemon juice. Toss to mix. Add caramelized onion and garlic mixture.
Season with salt and pepper.
Can be made 2 days in advance. Keep refrigerated until serving.