In large pot of boiling salted water, blanch escarole until wilted; drain, chill under cold water and thoroughly drain, squeezing out excess moisture. Chop and set aside.

In 11- to 12-inch/28 to 30 cm ovenproof skillet, melt butter over medium-high heat; add shallots and fry until softened but not browned, 2 to 3 minutes. Add ham; sauté until edges begin to brown. Stir in escarole and half of the salt; fry, stirring often, until escarole is tender, about 5 minutes. Sprinkle with flour; fry, stirring, for 2 minutes. Stir in milk; simmer for 2 minutes. Remove from heat; stir in cheese.

Beat together egg yolks, pepper and remaining salt; stir into skillet until thoroughly mixed. In large bowl, whisk egg whites until in soft but not dry peaks; fold into escarole mixture until no streaks of egg white remain. Bake in centre of 400 F/200 C oven until golden and set in centre, 20 to 25 minutes.

Makes 6 to 8 servings.

Variation
Vegetarian version: Omit ham or, if desired, replace with 3 cups/750 mL finely diced mushrooms and pinch salt, adding to skillet after shallots are softened and cooking until any liquid from mushrooms has evaporated and mushrooms are lightly browned. Then complete recipe as above.