Toss together first 3 ingredients in a shallow roasting pan; stir in 1/2 teaspoon salt and 1/2 teaspoon pepper.
Bake at 450°, stirring occasionally, for 30 to 40 minutes or until tender. Remove from oven; set aside.
Bring chopped peach and nectar to a boil in a saucepan. Cover, reduce heat, and simmer 5 minutes; let mixture cool.
Process peach mixture in a food processor or blender until smooth, stopping to scrape down sides. Pour into a bowl; whisk in remaining 1/4 teaspoon salt and 1/4 teaspoon pepper, shallot, mustard, lemon juice, and 1 tablespoon oil.
Drizzle roasted vegetables with 1/4 cup peach mixture, and toss gently.
Drizzle mixed greens with 1/3 cup peach mixture, and toss gently; divide greens among 6 individual plates, and top evenly with roasted vegetables.