Krämig svamp lasagne

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1. 1
   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.
2. 2
   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.
3. 3
   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.
4. 4
   Combine 1 large egg, 6 oz. Parmesan, finely grated (about 3 cups), and 2 cups whole-milk fresh ricotta in a medium bowl; season cheese mixture with salt and pepper. Spread about ¾ cup sauce across the bottom of a 13x9" glass or ceramic baking dish. Top with an even layer of noodles (about 3½). Cut in half lengthwise as needed to puzzle in tightly. (If noodles are sticking together, rinse under cool running water to help unstick.) Spread a heaping ⅓ cup cheese mixture over noodles, scatter a heaping ⅓ cup mushrooms evenly on top, and dollop a heaping ⅓ cup sauce on top. Starting with noodles, repeat layers, using more or less of various components depending on how quickly you deplete them. Spread final sauce layer (about ½ cup) thinly and evenly over mushrooms, then sprinkle 8 oz. low-moisture mozzarella, coarsely grated (1⅓ cups) evenly on top.
5. 5
   Bake lasagna until edges are golden brown and bubbling, 40–50 minutes. If you’d like additional color on top, heat broiler and broil just until center is browned (keep a close eye on lasagna to prevent burning), 1–3 minutes, depending on your broiler. Let lasagna sit 15 minutes before serving. Do Ahead: Lasagna can be assembled 1 day ahead. Cover and chill. Baking time will be 5–10 minutes longer.