Cook potatoes in pot of boiling salted water (or steam and rinse them) until just tender, about 20 minutes. Drain and cool. Cut potatoes in half.
Melt butter in heavy large saucepan over low heat. Add flour; whisk until mixture bubbles and thickens (do not brown), about 3 minutes. Gradually whisk in milk. Bring to simmer, whisking often (sauce will be thick). Simmer until very thick, stirring often, about 10 minutes. Mix in cream and nutmeg. Season with salt and pepper. Mix in potatoes. (Can be made 1 day ahead. Cover; chill. Bring to simmer before continuing, stirring often.) Sauté onion lightly, then mix onion and peas into sauce. Cook over medium-low heat until vegetables are heated through, stirring often, about 8 minutes.
Bon Appétit, November 1996
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