Melt the butter in a large saucepan or Dutch oven over a low/medium heat. Thinly slice the onion and the garlic. Add the onion and garlic to the pan with the melted butter. Stir well and sweat gently until softened and fragrant. This will take roughly 2-3 minutes.
Add the stock to the saucepan along with a pinch of nutmeg. Stir well. Bring up to a gentle boil and simmer for 2-3 minutes. You may use a chicken stock or any other stock or broth you prefer. You may also add salt and pepper if desired, depending on the strength of seasoning in your stock.
Roughly chop the lettuce. Add the lettuce to the saucepan/Dutch oven and stir. Let the lettuce cook gently in the simmering stock until only just wilted.
Remove the soup from the heat. Add the cream cheese. Carefully use a hand stick blender to purée the soup until velvety smooth. Alternatively, you may let the soup cool a little then carefully transfer to a free-standing blender to mix, working in batches if needed. Do not pour boiling hot soup directly into the blender.
Stir the heavy cream into the soup until well combined. If needed, return the soup to the stove to gently warm through. Serve the soup hot with sliced low carb toast or buttered Keto bread rolls.