To make the celeriac: Peel and cut the celeriac into roughly 1-inch/2.5 cm pieces. Put the lemon juice, milk, and bay leaf in a large pot, pour over 8-cups/2 liters water. Add the salt, bring to a boil, and simmer until the celeriac is very tender.
Drain, reserving a cup/250 ml of the cooking liquid. Remove the bay leaf. Stir the butter and cream into the celeriac, then puree until very smooth in a blender, adding as much of the reserved cooking liquid as needed to achieve a soft, smooth consistency. Season with pepper and salt, and keep warm.
To make the hens: Preheat the oven to 450 degrees F. Season the hens well with salt and pepper and place in a single layer in a baking dish. Tuck in the garlic, bacon, and herbs. Scatter over the juniper berries. Pour the beer in the side of the dish.
Bake 15 minutes. Turn the pieces and bake another 15 minutes. Turn again and bake 15 minutes longer. By now the meat should be cooked and sticky with a slightly caramelized sauce. If the hens are done, but there is still too much sauce, simply keep the hens warm while you boil the sauce down a little. Serve the hens with the sauce - herbs, juniper and all - poured over.
For the plate: Plate the celeriac puree and top with a piece of hen and enjoy!