Melt butter in a pot and sauté onions and carrots for 3 minutes. Add garlic, apples, water and bay leaf. Bring to a boil and lower the heat to simmer until the carrots are tender, about 15 minutes. Discard the bay leaf and puree soup in a blender, taking the necessary precautions, until very smooth. Season with salt and blend for a few more seconds until very smooth. Meanwhile, mix together the plain yogurt, chopped fresh dill and lemon juice.