Place spinach, with just water clinging to leaves, in large pot over medium-high heat; cover and cook, stirring once, until wilted, 4 to 5 minutes. Drain, refresh under cold water and drain again, pressing out moisture. Mince and set aside.

Butter 8- x 3-3/4-inch/2.5L soufflé dish with 1 tsp/5 mL of the butter; sprinkle sides and base with 1 tbsp/15 mL of the cheese; set aside. Preheat oven to 425F/220C.

In saucepan, melt remaining butter over medium heat; cook shallot until soft, about 2 minutes. Stir in flour; cook, stirring, until flour no longer smells raw, about 1 minute (do not let flour brown). Whisk in milk, salt, pepper and nutmeg; cook, whisking constantly, until thickened, about 3 minutes. Remove from heat. Whisk in egg yolks and remaining cheese. Scrape into large bowl; stir in spinach.

In separate bowl, beat egg whites with cream of tartar until stiff peaks form; fold one-third into spinach mixture. Fold in remaining whites. Scrape into prepared soufflé dish. Place on middle rack of oven; reduce heat to 400°F/200°C. Bake until puffed and golden, 30 to 35 minutes.

Makes 4 servings.