1) Preheat oven to 230C/Gas 8.
2) In a large bowl, combine the carrots, courgette, squash, and red pepper. Toss with 2 tablespoons olive oil, salt and pepper. Mix well. Spread in 1 layer on a foil lined baking sheet and roast for 15 minutes, stirring occasionally. When done, set aside to cool.
3) Remove the corn kernels by standing the cob on a plate or in a wide shallow bowl. Set the blade of a small, sharp knife at the juncture of the kernels and cob. Slice downward to remove the kernels. Repeat this procedure, working your way around the cob, until all the kernels are removed. Set aside.
4) Cut the avocado in half lengthwise and remove pit, leaving the skin on. Sprinkle with 1 tablespoon of lemon juice to prevent avocados from discoloring.
5) In a large bowl, mix roasted vegetables, corn, remaining lemon juice, the remaining 1 tablespoon olive oil, and fresh herbs. Taste and reseason with salt and pepper, if needed.
6)Place each avocado half on a small bed of mixed greens or arugula, if using. Top each avocado half with a generous serving of the roasted vegetable salad. Sprinkle lemon zest on top and serve.