Place the oil in a pot set over medium heat. Add the onion and celery and cook 3 to 4 minutes to soften. Add the stock, squash, quinoa and tomatoes and bring to a slow simmer. Simmer 15 to 18 minutes, or until the squash and quinoa are tender. Stir in the pesto; season with salt and pepper and serve.
Note: Chunks of cleaned banana squash are sold in the produce department.