Step 1

Preheat oven to 400ºF.

Step 2

Grease a 13"x9" baking dish with butter. Set it aside.

Step 3

Bring a large pot of water to a boil. Once boiling, add salt and the linguine. Cook until 1–2 minutes under al dente. Reserve ¼ cup pasta cooking water before draining the pasta.

Step 4

Place the pancetta into a wide pot. Turn the heat to medium. Cook, occasionally turning, for 5–6 minutes until the fat renders and the pancetta is crispy. Transfer to a bowl.

Step 5

Add the mushrooms to the pancetta fat and cook for 8–10 minutes. Add the shallots and cook an additional 5 minutes. Season all over with salt and pepper. Stir in the fresh thyme and toss to coat. Cook for 1 minute.

Step 6

Turn the heat to medium-high. Add the pancetta back to the pot. Once everything in the pot begins to sizzle, pour in the sweet vermouth and bring it to a boil. Reduce heat and simmer for 5 minutes until reduced by half. Transfer to a bowl.

Step 7

Melt 4 tablespoons of butter into the pot. Once bubbling and frothy, add the flour. Cook for 1–2 minutes, stirring the flour into the butter, until fragrant and golden.

Step 8

Add the chicken stock in ½-cup increments, whisking after each addition to form a slurry. Continue adding the stock in increments until completely incorporated and smooth.

Step 9

Pour in half the cream and whisk it into the mixture. Add the remaining half and whisk until smooth. Supplement with more water or milk if you like a looser sauce. Season with salt, nutmeg, and white pepper.

Step 10

Add the grated gruyère and ⅓ cup grated Parmigiano Reggiano and stir until completely melted into the sauce.

Step 11

Fold in the bowl of mushrooms, the diced turkey, ½ cup minced parsley, and the frozen peas, and cook until the peas are warm. Taste and season once more to your preferences.

Step 12

Add the cooked pasta to the sauce and toss to coat for 1 minute, adding the pasta cooking water only if needed. Turn off the heat.

Step 13

Pour the pasta and sauce into the prepared baking dish.

Step 14

Transfer to the oven for 10 minutes.

Step 15

As the tetrazzini bakes, melt the remaining 2 tablespoons of butter in a small skillet. Add the challah cubes and cook in the butter until golden brown, about 3–5 minutes. Turn off the heat and transfer to a food processor. Pulse until blended into a fine meal. Transfer the breadcrumbs to a bowl.

Step 16

Add the remaining 2 tablespoons grated Parmigiano Reggiano and the remaining ½ cup minced parsley to the breadcrumbs. Season with a pinch of salt.

Step 17

Remove the tetrazzini from the oven and cover it with breadcrumbs. Transfer back to the oven for an additional 10–15 minutes.

Step 18

Once the tetrazzini is bubbly, and the breadcrumbs are browned, remove them from the oven. At this point, you can broil the top for 1–2 minutes if you like. Otherwise, let stand for 5 minutes before serving.

Step 19

Spoon the tetrazzini into shallow bowls. Garnish with more fresh parsley if you like. Enjoy!