To make the b?chamel sauce, heat the butter in a saucepan over medium-low heat. Once melted, add in the flour gradually and whisk for 2-3 minutes.

Increase heat to medium-high and slowly add the milk, whisking continuously to remove lumps. Bring to a simmer, then reduce heat to medium-low. Cook for 7-10 minutes until thickened. Add nutmeg, salt, and pepper. Adjust seasoning if necessary.

Cook lasagna sheets in salted water until very al dente (about 2 minutes less than package instructions), then shock in cold water or an ice bath. Lay the sheets flat on parchment paper or clean kitchen towels.

Saut? the onion in 3 tablespoons olive oil over medium-low heat until soft (about 3-4 minutes). Add garlic and cook for 2 minutes until fragrant.

Clean the spinach under cold water. Add it to the pan with the onions and garlic. Cover and let steam until wilted (about 2-3 minutes). Place spinach in a wire mesh strainer and press to remove excess water.

Chop the spinach and add it to a large bowl with ricotta cheese. Mix in grated Parmesan, 2/3 of the mozzarella, and season with kosher salt. Adjust salt to taste. Then, mix in the 2 beaten eggs.

Spread a thin layer of b?chamel sauce on the bottom of a 10 by 15 or larger baking pan.

Spoon 3-4 tablespoons of the spinach ricotta mixture into the center of each lasagna sheet. Roll the sheets and place seam side down in the baking pan. Repeat with remaining spinach mixture.

Pour the remaining b?chamel sauce on top, leaving the ends of the rolls exposed. Sprinkle with the remaining mozzarella.

Cover with foil (ensuring it doesn't touch the sauce and cheese) and bake at 400?F for 25 minutes. Remove foil and broil for the last 2 minutes to brown the top.

Serve garnished with more Parmesan cheese and fresh minced parsley.