Heat the oil in a casserole dish and lightly brown the chicken legs.
Meanwhile, slice the shallots, garlic, celery and leek. Peel the kohlrabi then cut it into 3 cm cubes. Roughly chop the carrots.
Add the chopped leek, celery and shallot to the pot.
When the chicken is light brown all around, and the onion and celery have softened, add the garlic and thyme. Stir for 30 seconds, then add the carrot & kohlrabi pieces. Pour in just enough stock to cover the veggies.
Add the bay leaf, bring to a boil, then cover the dish with a lid.
Turn down the heat and gently simmer for about 40 minutes.
Add the drained butter beans and cook for another 5-10 minutes, until you get a thick sauce.
Stir in the lemon juice and zest, and scatter the chopped parsley on top.