Preheat oven to 450 degrees. combine grapes, 2 tsp olive oil and vinegar and place on a baking sheet lined with foil. Bake for 10 minutes; remove from oven and let stand for 5 minutes. Toss grapes, walnuts, lemon juice, remaining olive oil, salt, pepper and greens in a salad bowl until well-mixed and sprinkle with feta cheese.