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1
Preheat the oven to 300u00b0 F. Stir together the breadcrumbs, olive oil, 1/8 teaspoon salt, and a few grinds of pepper. Pour onto a rimmed baking sheet and toast for about 10 minutes, stirring once, until breadcrumbs are fragrant and golden brown. Set aside. Bring a pot of water to a boil for the pasta.
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2
In a large heavy skillet over medium heat, melt 3 tablespoons of butter. Add shallot, celery, and mushrooms with a generous pinch of salt and cook until the vegetables soften and start to brown, about 5 minutes. If they start to brown too quickly, add a little water to the pan.
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3
Stir in the flour and cook, stirring constantly, for about 1 minute, until the flour softens and melts. Turn up heat to medium and add the milk, whisking to break up any flour lumps. Stir constantly until sauce is thickened and bubbly, about 4 minutes. Turn heat down to low and season well with salt and pepper to taste. Add lemon zest, nutmeg, cream, and Parmesan and stir well. Keep warm.
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4
Next, cook the pasta. While the pasta is cooking, add the peas and tuna to the cream sauce and stir through. When the pasta is al dente, drain it, saving some of the cooking water. Add the pasta to the sauce and stir. If the sauce seems thick, add a little of the reserved cooking water. Taste for seasoning, then add lemon juice and parsley and stir again. Serve immediately, sprinkling each plate with some of the breadcrumbs.
