Top and tail the leeks, then cut into thick slices (about 1 inch). Rinse thoroughly in a colander under cold water. Cut the potatoes into roughly 1 inch cubes, and dry on paper towels. Melt the butter in a large saucepan (med. heat). Add the leeks and sprig of thyme. Cover and cook 4 miniutes. Add the potato pieces and just enough cold water to cover the vegetables. Re-cover the pot and cook on low heat for 30 minutes. Pour in the heavy cream and season with salt and pepper to taste. Cover and simmer for 30 minutes longer. Remove the thyme stem. Serve.
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