Melt the butter in a medium-sized pot over medium-high heat
Chop the onion roughly and add it to the pot with the butter, along with the salt
Peel and chop the rutabaga and apple and add to the pot
Allow the vegetables to sauté for 3-5 minutes, then add the beer (if using), along with the stock, rosemary, and bay leaf.
Bring to a boil, then reduce the heat to medium and simmer for 15-20 minutes or until the rutabaga and apple are fork-tender
While the soup is simmering, grate the cheese and set aside.
Remove the pot from the heat, remove the bay leaf, and blend the soup until smooth using the blending implement of your choice. For a smoother soup, use a high-powered blender like a Vitamix.
Add the cheese to the soup a small handful at a time, and whisk or blend in between additions.
If the soup is too thick, add stock or water until you reach the desired consistency.
Taste for seasoning and add more salt if necessary, and black pepper if you like.