Heat oven to 400°F. Lightly oil two baking sheets. Cut zucchini and summer squash lengthwise in half, then cut each half crosswise into large chunks (about 1 1/2" thick). In medium bowl, toss zucchini and squash with chopped pecans, 2 tsp. olive oil, and 1/4 tsp. salt. Spread mixture in a single layer on one baking sheet. Set aside.
Cut red onion into 1" thick wedges and place in same bowl. Cut tomatoes lengthwise into quarters and add to onion. Toss onion and tomatoes with remaining 2 tsp. oil and 1/4 tsp. salt. Spread mixture in a single layer on second baking sheet.
Roast all vegetables 5 minutes. Turn vegetables and pecans and roast 4 - 5 minutes longer or until most of the vegetables are just tender and lightly browned-do not overcook. Combine all the vegetables and chopped pecans in a large bowl, sprinkle with vinegar and gently stir.
To serve, divide salad greens among plates. Top each with a mixture of vegetables and chopped pecans. Pour any liquid in bowl over vegetables. Garnish each serving with 3 pecan halves and, if desired, Parmesan cheese.