Grind the juniper berries and rosemary in a spice grinder or mortar & pestle until very fine.
Melt the butter in a small saucepan over medium heat. Stir in the flour and cook until it turns a light golden brown.
Meanwhile, stir the milk, yoghurt, and lemon juice together in a separate bowl.
Once the roux is golden brown, add the milk mixture tablespoon by tablespoon. Whisk continuously until very smooth, adding more milk if the mixture is too thick. Bring to a simmer and cook 10 minutes and remove from heat. Season with salt, rosemary, and juniper.