Position a rack in the center of the oven and preheat to 350°F.,

In a large pot over medium-high heat, melt the butter. When the foam subsides, add the onion and cook, stirring occasionally, until soft, about 5 minutes. Add the mushrooms and bell pepper, and cook, stirring occasionally, until tender, about 5 minutes. Add the flour and cook, stirring continuously, until no streaks of flour remain, about 2 minutes. Pour in the stock and milk and cook, stirring occasionally, until the sauce has thickened, about 5 minutes. Turn the heat to medium and stir in the turkey, half the cheddar, the parmesan, and sherry. Continue cooking, stirring occasionally, until the cheese has melted, about 8 minutes more.,

Meanwhile, bring a generously salted pot of water to a boil. Add the spaghetti and cook according to package instructions until al dente. Drain, then add the spaghetti to the pot with the sauce and stir in the peas. Season to taste with salt and black pepper, then spread evenly into a 9- by 13-inch baking dish and top with the remaining cheddar.,

Bake until golden and bubbly, about 35 minutes. Set aside to rest for 5–10 minutes before serving.