Fill the 6-quart pot with 4 quarts of water, add 2 teaspoons of salt, and bring it to a boil.

Dice the onion and celery, slice the mushrooms, mince the garlic, and drain the tuna in a colander, pressing gently with a fork. Shred the cheddar.

Mix 1/2 cup of cheddar with the panko and melted butter in a small bowl for the topping.

Boil the noodles for 5 minutes, stirring once after the first 2. Drain without rinsing, until the noodles bend easily but still feel firm in the center.

Melt 4 tablespoons of butter in the Dutch oven over medium heat. Cook the onion and celery for 5 to 6 minutes, stirring often, until the onion looks soft and see-through.

Add the mushrooms and cook for 6 to 8 minutes, adding the garlic for the last 30 seconds, until the pan looks nearly dry and the mushroom edges are deep brown.

Sprinkle the flour over the vegetables and stir for 1 minute, until the mixture looks like damp sand with no dry flour visible.

Pour in the milk 1 cup at a time, whisking until smooth after each addition. Simmer over medium heat for 4 to 5 minutes, whisking often, until the sauce coats a spoon and a finger drawn through it leaves a clean line.

Move the pot off the heat. Stir in the sour cream, 1 cup of cheddar, lemon juice, 3/4 teaspoon salt, pepper, thyme, paprika, and mustard until the sauce looks glossy and pale gold with no oily streaks. Taste it, and it should be slightly too salty on its own.

Fold in the noodles, tuna, and frozen peas, breaking the tuna into flakes, until every noodle looks coated and the peas show bright green.

Scrape the mixture into the greased dish and smooth the top. Scatter the panko topping evenly until no filling shows through.

Bake uncovered on the middle rack at 375°F for 25 to 30 minutes, until the edges bubble and the top is deep golden brown. Rest for 5 minutes so the sauce thickens and the casserole slices into clean squares.