Begin the recipe by preheating the oven to 350 degrees F. and grabbing a square 8x8-inch baking dish. Set aside.
Using a large skillet over medium-high heat, add the 1 pound ground beef and 1 cup chopped onion. Cook until it has browned. Add in the 1 clove minced garlic and cook until fragrant, about 1 minute.
Drain off any excess fat/liquid and stir in the 24 ounce jar of pasta sauce. Cover the sauce and adjust the heat to medium-low, allowing it to simmer while you prefer the rest of the ingredients.
In a medium size bowl, add the 15 ounces ricotta cheese, ½ cup grated Parmesan cheese, 1 large egg, ½ tablespoon Italian seasoning, ½ teaspoon salt, and ½ teaspoon ground black pepper. Stir to combine.
Spoon a third of the meat sauce into the bottom of the baking dish. Spread it out in an even layer. Top with a layer of 2 to 3 oven ready lasagna noodles.
Top the noodles with half of the leftover meat sauce and spread it out in a single layer. Add half (½ cup) baby spinach and half of the ricotta cheese mixture. Evenly sprinkle half (1 cup) of the shredded mozzarella cheese.
Top the mozzarella cheese with the rest of the lasagna noodles. Followed by a layer of the rest of the meat sauce, (½ cup) baby spinach, ricotta cheese mixture, and (1 cup) mozzarella cheese.
Cover the baking dish with some aluminum foil and place that lasagna into the oven to bake for about 20 to 25 minutes, until the cheese has melted. Remove the foil and continue baking for an additional 5 to 10 minutes, it should be nice and bubbly.
Lastly, turn on the broiler to high and allow the top of the cheese to lightly brown. Watch it carefully though, you should only need about 2 to 5 minutes. Carefully remove it from the oven and allow it to cool for 5 to 10 minutes before cutting to serve warm. Enjoy!