This is red mustard.
These cool season crops grown under cover must be sown in August for an early spring crop , any later and it be quite some time before they can be harvested and greens like these are so valuable at this time of year. Spinach is also very good as is spring onions.
This will be an abundant source for salads in a few weeks, It really is too early to pick yet, but a tiny bit adds flavor to a wild salad mix.
So we ate the ground elder in the salad on Friday and it was delicious. Later on, the tougher leaves can be eaten cooked like spinach