In a heavy saucepan heat the butter and saute the onions slowly until golden brown. Season with salt and pepper.
Add the liquid and bring to a boil. Adjust the seasoning and add thyme. Cover and simmer for 30 to 40 minutes.
If you are using Stilton rind, grate it finely. Otherwise, chop the cheese. Add it half way through the cooking and stir well.
Serve with croutons.