Finely dice the onion into 1/4-inch pieces, mince the garlic, grate the parmesan cheese, and roughly chop the spinach into 1/2-inch ribbons. Preheat your oven to 400°F. Having everything prepped and ready before you start cooking ensures a smooth, efficient assembly process.

Heat a large skillet over medium-high heat and add the ground turkey, breaking it apart as it cooks. Once the turkey is mostly cooked through, add the diced onion and minced garlic, stirring frequently until the onion is softened and the turkey is fully browned, about 5-7 minutes total. Drain any excess fat from the pan, then season generously with salt and pepper. Set the cooked turkey mixture aside.

Bring a large pot of salted water to a rolling boil and add the lasagna noodles, cooking until just tender (usually 1-2 minutes less than package directions, as they'll continue cooking in the oven). Drain the noodles and lay them flat on a clean kitchen towel to prevent sticking. I find this prevents the noodles from tearing when layering.

In a medium saucepan, melt the butter over medium heat. Once melted and foaming, sprinkle in the flour while whisking constantly to create a smooth roux—cook this mixture for about 2 minutes, stirring frequently, to remove the raw flour taste. Gradually pour in the milk while whisking continuously to avoid lumps, then increase heat to medium-high and stir often until the sauce thickens enough to coat the back of a spoon, about 5-8 minutes. Whisk in the mustard and nutmeg, then remove from heat. I always add the nutmeg at the end so its delicate flavor isn't cooked away.

Add the 2 cups of white cheese and the freshly grated parmesan from Step 1 to the warm béchamel sauce, stirring until both cheeses are completely melted and the sauce is smooth and creamy. Taste and adjust seasoning with salt and pepper as needed.

Pour the warm cheese sauce from Step 5 into the cooked turkey mixture from Step 2, stirring gently until well combined. Set this turkey-cheese mixture aside.

Spread half of the turkey-cheese mixture from Step 6 into the bottom of a 9x13-inch baking dish. Layer 3 of the cooked noodles from Step 3 over the sauce, then dollop and spread the ricotta evenly across the noodles, followed by the spinach from Step 1. Arrange the remaining 3 noodles on top, then spread the remaining turkey-cheese mixture over them. Top with the 1.5 cups of white cheese. Bake uncovered at 400°F for 45-50 minutes until the cheese is bubbly and golden brown and the edges are slightly caramelized. Let the lasagna rest for 5 minutes before serving—this allows the layers to set so you get clean, intact portions.