Heat a large pot of water for noodles.
Preheat oven to 400˚F, rack at center.
For tuna, in large skillet, melt butter over medium heat. When it foams, add mushrooms and turn up heat a bit to brown, then lower back to medium, add celery, shallots or onion, garlic, leeks, thyme, salt, black pepper and white pepper. Soften a couple of minutes, stirring frequently, add sherry and let it absorb and reduce, add flour and stir again, stir or whisk in broth and cream, simmer to thicken, then add tuna and lemon juice.
For chicken, in large skillet, melt butter over medium heat. When it foams, add carrots, celery, shallots or onion, leeks, thyme, salt, black pepper and white pepper. Soften a couple of minutes, stirring frequently, add sherry and let it absorb and reduce, add flour and stir again, then stir or whisk in liquids, add peas and chicken and simmer to thicken a bit.
In either method, while sauce reduces, cook noodles 5 minutes in salted water, then drain and add to sauce. Transfer to casserole.
Combine crackers or crumbs, cheese, celery tops and parsley, sprinkle over casserole, and bake to lightly brown, about 7 to 10 minutes. Top with fried onions, if using.
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