Preheat the oven to 350°F (180°C) and line a large baking sheet with parchment paper.
In a microwave-safe bowl, combine the cream cheese, 2 cups shredded mozzarella, and Parmesan; microwave in 30-second intervals, stirring between, until mostly melted and smooth.
Whisk the eggs into the melted cheese mixture until smooth to form the keto lasagna batter.
Spread the batter evenly onto the prepared baking sheet and bake for 15 minutes, or until edges are set and surface is firm; remove and let cool, then slice into six even lasagna sheets.
Meanwhile, heat a little oil in a non-stick skillet over medium heat. Sauté the chopped onion and minced garlic for about 5 minutes, until softened.
Add the ground beef to the skillet and cook about 3 minutes, breaking it up, then stir in the marinara sauce and cook another 5 minutes; remove from heat.
Line an 8x8-inch baking pan with foil. Place two lasagna sheets in the bottom, spread 1/2 cup ricotta over them, add one-third of the meat sauce, and sprinkle with 1/2 cup shredded mozzarella. Repeat layers until all sheets and fillings are used.
Bake the assembled lasagna at 350°F (180°C) for 30–35 minutes, until the top is golden and sauce is bubbling.
Remove from the oven and let rest for 5 minutes before slicing and serving.