Preheat oven to 400°F. Place a rack in the lower third of the oven. Butter a 5x9-inch loaf pan or a 7-inch soufflé dish. Coat with flour.

Melt butter in a medium saucepan over medium heat. Add bay leaf, nutmeg, and chard. Cook until moisture is released and chard is tender, about 5 minutes.

Sprinkle flour over chard and stir, scraping up flour as needed. Continue cooking, stirring and scraping all the time, for about 5 minutes.

While stirring, slowly add milk, and continue cooking until thickened and bubbling. Add salt and cook for 1 additional minute, then remove from heat and remove bay leaf. Season with additional salt and pepper as needed.

In a large bowl, mix together egg yolks and cheese. While stirring rapidly, slowly pour in hot sauce. Continue stirring until smooth, about 1 minute. Set aside.

In the bowl of a mixer fitted with the whisk attachment, beat egg whites on medium until foamy. Add cream of tartar and increase the mixer speed to high. Beat until the egg whites hold stiff but shiny peaks.

In three additions, fold egg whites into cheese and egg sauce. Scrape into prepared pan and bake for 30 minutes, or until golden brown, puffed, and a skewer inserted into the center comes out clean.

Serve at once.