Welcome to Laura Gray via Bloglovin. Last year whilst out and about on holiday, we popped into Ryton Organic Garden Centre, see here, and purchased a few packets of seeds.
One of them was Asparagus Lettuce. It is a cooler weather crop, sown in Spring or Autumn. Early leaves are picked and eaten in salad. Later leaves are slightly tougher and are picked and used like spinach, from the growing stem.
Eventually, you end up with a tall, mainly leaveless stem, which is then harvested. It can be used like asparagus but apparently has a pea-like/celery flavour from what I have read. You can send for the seeds from here.
I sowed it too late last year so barely got a stem. However, an earlier sowing this year has produced its first stage of salad leaves: