1. In a large skillet, melt 1 tablespoon butter over medium-high heat. Add onion and cook, stirring, until softened and lightly browned, 2 to 3 minutes. Season with salt and pepper and transfer to a large bowl; set aside.

2. Reduce heat to medium-low and add 2 tablespoons butter. When butter has melted, add mushrooms, season with salt and pepper, and cook, stirring, until softened and browned, 25 to 30 minutes. Transfer to bowl with onions; set aside.

3. Preheat oven to 350 degrees. Spray a 3-quart baking dish with nonstick cooking spray.

4. In a medium saucepan over high heat, toast kasha, stirring constantly, until fragrant and toasted, about 1 minute. Add egg and stir to coat. Add chicken stock, 2 teaspoons salt, and 1/4 teaspoon pepper; bring to a boil. Reduce heat to a simmer, cover, and cook until all liquid is absorbed, about 10 minutes.

5. Transfer kasha to bowl with onions and mushrooms. Add pasta and remaining 4 tablespoons butter; stir until butter is melted and mixture is well combined. Transfer mixture to prepared baking dish and bake until lightly browned, 25 to 30 minutes. Sprinkle with parsley before serving.