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For the Casserole: Position rack in middle of oven. Preheat oven to 375°F/190°C. Coat a 2-quart (2 L) casserole dish with nonstick spray; set aside.
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Cook the pasta in a large pot of salted water until a very firm al dente. Right before draining, add the peas to the boiling water; drain pasta and peas and set aside.
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Melt the butter over medium heat in a large skillet, add the leeks and scallions and sauté for about a minute, then stir in the mushrooms and cook, stirring often, until softened, about 3 to 4 minutes total.
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Whisk in flour and cook, stirring often, for a minute or two to lightly cook the flour, then slowly whisk in the stock and milk until smooth. Simmer for a few minutes, whisking often until sauce thickens; whisk in soy sauce. Sprinkle in the cheese and whisk well, again cooking until smooth and incorporated. Add tuna and break it up gently. Remove from heat. Taste and season with salt and pepper. Fold in the pasta and peas and scrape the mixture into your prepared casserole dish.
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For the Topping: Toss together the breadcrumbs, cheese, Garlic-Infused Oil and Parmesan cheese in a small bowl until well combined. Sprinkle evenly over the casserole.
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Bake for about 20 to 25 minutes or until casserole is hot and bubble and topping is golden brown. Serve immediately while the topping is nice and crispy and the inside is hot and creamy. A green salad is a great side.