2 tbsp. olive oil
6 tbsp. butter
3 cloves garlic, 1 crushed and 2 finely chopped
1 cup homemade breadcrumbs or panko
2 thin (1/4 to 1/2 inch thick) tuna steaks (3/4 to 1 lb. total)
Salt and pepper
¼ lb. shiitake mushroom caps or white mushroom caps, thinly sliced
2 large shallots, finely chopped
1 stalk celery with leafy top, finely chopped
1 tbsp. fresh thyme or 1 tsp. herbes de Provence
1 bay leaf
About 3 tbsp. flour
½ cup dry white wine
1 cup chicken stock
½ cup heavy cream
1 rounded tsp. Dijon mustard
12 oz. egg tagliatelle or extra-wide egg noodles
About 3/4 cup freshly grated Parmigiano-Reggiano or Parmesan
½ cup fresh flat-leaf parsley tops, finely chopped
2 tsp. lemon zest, plus juice of 1/2 lemon (about 2 tbsp.)
In a large skillet, heat 1 tbsp. olive oil, one turn of the pan. Melt in 2 tbsp. butter. Add the crushed garlic and swirl for a minute. Add the breadcrumbs. Cook, stirring often, until toasted and golden. Transfer to a bowl. Wipe out the skillet.
Bring a large pot of water to a boil for the noodles.
Heat the same skillet over medium-high to high. Season the tuna with salt and pepper. Add the remaining 1 tbsp. oil, one turn of the pan, to the skillet. Add the tuna. Cook just until cooked through, about 2 minutes per side. Place the tuna on a plate.
Reduce heat a little. Add the remaining 4 tbsp. butter to the skillet. When it melts, add the mushrooms. Cook, stirring occasionally, until browned, 2 to 3 minutes. Add the shallots, celery, thyme, bay leaf, and the chopped garlic; season with salt. Cook, stirring often, until the shallots soften, 2 to 3 minutes more. Whisk in the flour. Cook, whisking often, until the flour is absorbed, about 1 minute. Whisk in the wine, then the stock, and bring to a bubble. Cook the sauce, whisking often, until it thickens a bit, about 4 minutes. Reduce heat to low. Stir in the cream and Dijon. Season the sauce with white pepper and salt.
Using a fork, flake the tuna into small pieces.
Salt the boiling water and add the noodles. Cook until 1 minute shy of the package directions. Reserve 1/2 cup of the starchy cooking water, then drain the noodles.
Mix the cheese, parsley, and lemon zest into the breadcrumbs.
Add the tuna and noodles to the sauce. Toss, thinning the sauce with the starchy cooking water if it gets too thick. Stir in the lemon juice and remove the bay leaf.
Serve in shallow bowls. Top with the garlicky breadcrumbs.