How To Prepare
To prepare wild rice: Combine water and rice in a small heavy saucepan; bring to a boil. Cover, reduce heat to maintain a simmer and cook until the rice is tender with a slight bite, 40 to 50 minutes. Drain; you’ll have about 1 1/2 cups cooked rice.
To prepare frittata: About 30 minutes after you start cooking the rice, beat egg whites in a large bowl with parsley, 1/4 teaspoon pepper and nutmeg.
Position rack in upper third of oven; preheat broiler.
Heat oil in a 10-inch ovenproof skillet, preferably cast-iron, over medium heat. Add onion and the remaining 1/4 teaspoon pepper; cook, stirring, until softened, about 3 minutes. Stir in rosemary, then add mushrooms and cook, stirring frequently, until they release their liquid and the pan is dry, 6 to 8 minutes. Reduce heat to medium-low; stir in the rice.
Pour the reserved egg mixture evenly over the rice and vegetables. Partially cover and cook until set around the edges, about 5 minutes. Place the pan under broiler and broil until the eggs are set and the top is nicely browned, about 2 minutes. Let stand for 5 minutes before serving.
TIPS & NOTES
Make Ahead Tip: Prepare the rice (Step 1), cool and store airtight in the refrigerator for up to 3 days.
Ingredient note: Regular wild rice takes 40 to 50 minutes to cook. To save time, look for quick-cooking varieties, which can be on the table in less than 30 minutes, or instant wild rice, which is done in 10 minutes or less.