Preheat the oven to 400˚. Toss the vegetables and olive oil in a large roasting pan until evenly coated, and sprinkle lightly with salt and pepper.
Roast the vegetables for a total of 30 minutes, gently stirring after 15 minutes so that they cook evenly. Set the vegetables aside and allow them to cool for 10–15 minutes.
When you are ready to assemble the sandwiches, spread a generous amount of Celery Root and Red Pepper Spread on one slice of bread per sandwich. Layer on a variety of the roasted vegetables and top with a few leaves of fresh basil. Spread mayo on the other slice of bread before topping the sandwich, and serve.