1. Place potato in a pan; cover with water. Bring to a boil. Reduce heat; simmer 8 minutes or until tender. Drain. Press potato through a ricer or food mill into a bowl.
2. Place mushrooms, leek, and thyme in a food processor; pulse until finely chopped. Heat a large skillet over medium heat. Coat pan with cooking spray. Add mushroom mixture; cook 6 minutes, stirring occasionally. Add mixture to potato. Add Gruyère, 2 tablespoons Parmigiano-Reggiano, salt, pepper, and egg yolk; stir until blended. Shape mixture into 8 (2-inch) round patties.
3. Place flour in a shallow dish. Combine egg white and 2 teaspoons water in a shallow dish, stirring with a whisk. Combine remaining 2 tablespoons Parmigiano-Reggiano and panko in a shallow dish. Dredge patties in flour. Dip in egg mixture; dredge in panko mixture. Heat a large nonstick skillet over medium-high heat. Add oil; swirl to coat. Add patties; cook 4 minutes on each side or until golden.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.