In a heavy pan, melt the butter, add the onions, and cook gently for 2 minutes. Do not allow them to color. Add the cucumbers and sauté for 2 minutes, then stir in the stock, mace, some salt, and a good grinding of pepper. Cover and stew gently for 6-10 minutes or until the cucumbers are soft but not mushy.
Mix the mustard and lemon juice together and stir into the cucumbers. Allow to b