For the casserole: Preheat the oven to 375degrees F. Butter a 13- by 9-inch ovenproof dish or lasagna dish and set aside.
Cook the egg noodles in salted boiling water according to package directions for al dente, about 8 minutes. Drain and immediately place the noodles in a large bowl filled with ice water to stop the cooking. Once cooled, drain and then pour the pasta into a large bowl with the tuna, peas and both cheeses. Toss to combine.
In a large pan with straight sides, add the 2 tablespoons butter, the olive oil, onions and thyme. Season with a sprinkle of salt and pepper and cook on medium heat until the onions are tender and translucent, about 5 minutes. Add the mushrooms, season with salt and cook gently until tender and darkened, about 5 minutes more. Add the Worcestershire sauce and horseradish, and then sprinkle the flour over the entire pan. Stir to help the flour soak into the vegetables and cook a minute more to lose the flour taste. Raise the heat to medium-high and add the chicken stock. Cook until slightly thickened, about 5 minutes, and then slowly stir in the heavy cream. Simmer until the cream is thickened slightly, about 4 minutes more. Taste and season with salt if needed. Pour the mushroom sauce over the prepared noodles in the large bowl and quickly stir to combine. Immediately pour into the prepared dish.
For the topping: In a small bowl, combine the breadcrumbs and olive oil. Season with a sprinkle of salt and a grind or two of pepper. Stir until the crumbs soak up the oil evenly. Sprinkle the breadcrumb mixture evenly over the top of the dish and place, uncovered, in the oven until the sides are bubbly and the top is golden brown, about 35 minutes. Let it rest 5 minutes before serving.
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