Creamy Mushroom Lasagna

Place a rack in middle of oven; preheat to 375°. Cook 1 lb. dried lasagna in a large pot of generously salted water, stirring occasionally, until about 3 minutes shy of al dente (it should be about two thirds of the way cooked and still slightly too hard to eat), about 5 minutes. Drain pasta, rinse under cold running water until cool to the touch, and set aside.

Meanwhile, melt 6 Tbsp. unsalted butter in a large Dutch oven or other heavy pot over medium-high heat. Add 2 lb. crimini mushrooms, trimmed, thinly sliced, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ½ tsp. freshly ground pepper. Cook, stirring often, until mushrooms release their moisture, shrink, and brown, 15–20 minutes. Transfer to a large bowl; reserve pot.

Melt remaining 10 Tbsp. unsalted butter in reserved pot over medium heat. Cook 3 shallots, finely chopped, 8 garlic cloves, finely chopped, and 2 Tbsp. finely chopped thyme and cook, stirring often, until shallots are softened to an almost paste-like texture and mixture is fragrant, 7–10 minutes. Add ½ cup all-purpose flour, 2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt, and 1 tsp. freshly ground pepper and cook, stirring constantly, until onions are evenly coated in flour and no dry pockets remain, 1–2 minutes. Gradually pour in 5 cups whole milk, whisking constantly, and bring to a simmer. Cook, whisking constantly, just until thick enough to coat a spoon, 1–2 minutes. Stir in 2 Tbsp. Dijon mustard and 2 tsp. finely grated lemon zest. Taste sauce and season with more salt and pepper if needed.