Preheat oven to 425 degrees F. In 13x9 pan, toss potatoes and onion in 1 tablespoon olive oil, ½ teaspoon salt, pepper, and herbs. Roast 15 minutes. Add carrots, squash, peppers, and 2 tablespoons olive oil. Add salt and pepper to taste and half the lemon slices. Roast 45 minutes more, turning vegetables until tender. Garnish with lemon slices.