For the Topping: Adjust oven rack to middle position and heat oven to 350 degrees. Mix breadcrumbs, salt, and butter in small baking dish; bake until golden brown and crisp, 15 to 20 minutes. Cool to room temperature and mix with 1/4 cup grated Parmesan in small bowl. Set aside.

For the Filling: Increase oven temperature to 450 degrees. Heat 2 tablespoons butter in large skillet over medium heat until foaming subsides; add mushrooms and onions and sauté, stirring frequently, until onions soften and mushroom liquid evaporates, 12 to 15 minutes. Season with salt and ground black pepper to taste; transfer to medium bowl and set aside. Meanwhile, cook pasta in large pot of boiling, salted water until al dente. Reserve 1/4 cup cooking water, drain spaghetti, and return to pot with reserved liquid.

For the Sauce: Melt remaining 4 tablespoons butter in cleaned skillet over medium heat. When foam subsides, whisk in flour and cook, whisking constantly, until flour turns golden, 1 to 2 minutes. Whisking constantly, gradually add chicken stock. Adjust heat to medium-high and simmer until mixture thickens, 3 to 4 minutes. Off heat, whisk in sherry, Parmesan, nutmeg, 1/2 teaspoon salt, lemon juice, and thyme. Add sauce, sautéed vegetables, peas, and meat to spaghetti and mix well; adjust seasonings to taste. Turn
mixture into a buttered 13- x 9-inch baking dish (or other shallow, ovenproof dish of similar size), sprinkle evenly with reserved breadcrumbs, and bake until breadcrumbs brown and mixture is bubbly, 13 to 15 minutes. Serve immediately.