Creamy Mushroom Lasagna

Step 1

Place a rack in middle of oven; preheat to 375°. Cook1 lb. dried lasagnain 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.

Step 2

Meanwhile, melt6 Tbsp. unsalted butterin a large Dutch oven or other heavy pot over medium-high heat. Add2 lb. crimini mushrooms, trimmed, thinly sliced,1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and½ 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.

Step 3

Melt remaining10 Tbsp. unsalted butterin reserved pot over medium heat. Cook3 shallots, finely chopped,8 garlic cloves, finely chopped, and2 Tbsp. finely chopped thymeand cook, stirring often, until shallots are softened to an almost paste-like texture and mixture is fragrant, 7–10 minutes. Add½ cup all-purpose flour,2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt, and1 tsp. freshly ground pepperand cook, stirring constantly, until onions are evenly coated in flour and no dry pockets remain, 1–2 minutes. Gradually pour in5 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 in2 Tbsp. Dijon mustardand2 tsp. finely grated lemon zest. Taste sauce and season with more salt and pepper if needed.

Step 4

Combine1 large egg,6 oz. Parmesan, finely grated (about 3 cups), and2 cups whole-milk fresh ricottain 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 sprinkle8 oz. low-moisture mozzarella, coarsely grated (1⅓ cups)evenly on top.

Step 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.