Tuna Rice Puff

Bring the rice and water to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the rice is tender, and the liquid has been absorbed, 20 to 25 minutes.
Preheat an oven to 350 degrees F (175 degrees C). Beat egg whites until foamy in a large glass or metal mixing bowl. Lift your beater or whisk straight up: the egg whites should form a sharp peak that holds its shape; set aside.
Melt the butter in a large saucepan over medium-low heat. Whisk in the flour, salt, and pepper; stir until the mixture becomes paste-like and light golden brown, about 5 minutes. Gradually whisk the milk into the flour mixture, and bring to a simmer over medium heat. Cook and stir 5 minutes. Whisk the egg yolks in a bowl until broken, then whisk in the milk mixture a few tablespoons at a time until you have whisked in 1/3 of the milk mixture. Scrape the egg yolk mixture into the saucepan with the remaining milk, and whisk until smooth. Cook 2 more minutes, stirring constantly.
Remove the milk sauce from the heat, and stir in the rice, tuna, onion, and lemon juice. Gently fold in the egg whites until evenly blended, then pour into a 1 1/2 quart casserole dish.
Bake in the preheated oven until the casserole has puffed slightly, is golden brown, and has set in the center, about 40 minutes.