To make the jus, mix the beer with all the other ingredients up to and including the cornflour in a pan, bring to the boil and allow to cool a little. Rinse the chicken (inside and out) in cold water, pat dry, season. Place the chicken, breast side up, in a wide-bottomed, oiled casserole dish and pour over the jus.
Roast for approx. 10 mins. in the lower half of an oven preheated to 220°C. Baste the chicken with the jus. Reduce the heat to 180°C, cook the chicken for another hour, spooning over the jus 3-4 times during this time. The chicken is cooked when the juices run clear when pricked. Once cooked, turn the oven off and leave the chicken to rest for approx. 10 mins. with the oven door slightly open.
Place a steaming basket into a wide-bottomed pan, fill with water until it is just below the bottom of the basket, add the potatoes, carrots and kohlrabi, season with salt, cover and cook over a medium heat for approx. 20 mins. until soft. Remove the vegetables and place in a bowl.
To make the dressing, mix the mustard with all the other ingredients up to and including the shallot, season. Pour the dressing over the warm vegetables and mix well. Stir in the baby spinach shortly before serving.