Peel and thinly slice the onions. Cut the cucumbers in half lengthways, scoop out the seeds, and then slice the cucumber halves. Melt 2 oz / 60 g butter in a frying pan, and mix the rest with the flour to make beurre manié. Fry the onion in the melted butter for a few minutes until wilted, and then add the cucumber. Fry together for a few minutes more. Add the stock, wine and mace, then gradually stir in the beurre manié in small pieces. Shake the pan, then let all the ingredients stew together for a few minutes. Season and serve.