Bring a large pot of water to a boil over high heat. Season it with salt.
For the cream sauce:
Place the butter in a large saucepan over medium heat. Once it has melted, add the onion and garlic and cook for 3 minutes. Add the flour and cook for 3 more minutes, whisking constantly. Whisk in the broth, milk, salt, pepper, and basil. Bring the mixture to a simmer and cook, whisking frequently, for about 5 minutes, until thickened. Add the parmesan and cook, whisking constantly, for about 1 minute, until it melts into the sauce. Remove from the heat.
For the ricotta cheese mixture:
Place the spinach on a lint-free kitchen towel or a large piece of cheesecloth and squeeze out as much liquid as possible.
Combine the drained spinach, ricotta cheese, mozzarella, parmesan, parsley, salt, basil, pepper, and egg in a medium bowl. Stir well to combine.
To assemble the lasagna:
Cook the lasagna noodles according to the package instructions. Drain and rinse them under cold water.
Pour ½ cup of the cream sauce on the bottom of a 9x13-inch baking dish. Place 3 noodles side-by-side to cover the whole base of the dish. Spread half of the ricotta mixture over noodles. Place half of the chicken evenly over the ricotta mixture, then pour one third of the cream sauce over the chicken. Repeat by placing 3 noodles side-by-side in the dish and spreading the remaining ricotta mixture over noodles. Add the remaining chicken and one third of the cream sauce over the chicken. Top with the remaining 3 noodles and the remaining cream sauce. Sprinkle the top with the mozzarella.
Cover the dish with foil and bake for 25 minutes. Uncover, and continue baking for 20 minutes. For a golden brown top, broil on high for 2 to 3 more minutes. Let the lasagna sit for at least 15 minutes before serving.
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Notes
Squeeze the spinach well to remove any extra moisture before adding it to the ricotta mixture. The excess moisture can make the lasagna watery instead of lucious and creamy.
Use freshly grated cheese if possible. Pre-shredded cheese has anti-clumping agents, which means it doesn’t melt as nicely. It’s fine to use in a time crunch, but buying a block and shredding it on a box grater will be worth the little bit of extra effort!
Use a deep 9x13-inch baking dish. If you don’t have a deep one (2½” to 3” deep), place the baking dish on a rimmed sheet pan to catch any drippings that may bubble out.
To keep the foil from sticking to the cheese: Arrange 4 toothpicks, evenly spaced, in the lasagna before loosely covering it with foil to bake. Remove them after the foil is removed.
For a golden top, broil the dish on high for 2 to 3 minutes at the end of baking. Keep a close eye on it, as it can burn very quickly.
Let the chicken lasagna sit for 15 minutes to give it a chance to both cool and set up. It will stay hot for up to an hour. Take the resting time to make a batch of homemade garlic bread to serve with it!