Prepare the Sauce:

In a medium saucepan, melt 2 tablespoons butter over medium-low heat.

Add 4 teaspoons flour and whisk for 3 minutes until golden.

Gradually add 1 1/4 cups milk, whisking until thickened (about 3 minutes).

Season with 1/4 teaspoon salt, 1/8 teaspoon black pepper, and a pinch of nutmeg. Set aside.

Preheat the Oven and Prepare the Noodles:

Preheat oven to 450°F (230°C).

Boil salted water and add 1-2 tablespoons olive oil. Cook 12 lasagna noodles until al dente (8-10 minutes). Drain and arrange them in a single layer on a baking sheet.

Prepare the Ricotta Mixture:

In a bowl, mix ricotta, thawed spinach, 1 cup Parmesan, egg, 3/4 teaspoon salt, and 1/2 teaspoon pepper until smooth.

Assemble the Lasagna Rolls:

Butter a 13×9-inch baking dish and spread a bit of the sauce on the bottom.

Lay out 4 noodles and spread 3 tablespoons ricotta mixture on each.

Roll each noodle tightly like a jelly roll and place seam side down in the dish. Repeat with remaining noodles.

Pour marinara sauce over the rolls and top with mozzarella and remaining Parmesan.

Bake:

Cover with foil and bake for 20 minutes, or until bubbling.

Remove foil and bake for another 15 minutes until cheese is golden. Let rest for 10 minutes.

Serve:

Heat remaining marinara sauce in a saucepan. Serve lasagna rolls with sauce on the side.