Place an oven safe, large skillet (like cast iron) over medium-high heat and add 1 teaspoon of olive oil to the pan. Add the sausage and cook, breaking it up into smaller pieces as it cooks, until brown and cooked through. Turn the heat down to medium and add the onion to the pan along with 1/2 teaspoon of kosher salt. Cook, stirring often, until the onion is translucent.
In a medium bowl combine the ricotta, heavy cream, lemon zest, 1/2 cup of the parmesan, 1/2 teaspoon of salt and a few grindings of black pepper. Set aside.
When the sausage and onion are cooked scrape them into a medium bowl (do not clean out the pan you cooked them in, just set it aside on your stove top). Reserve 1/2 cup of the marinara sauce, then pour the rest of the bottle into the bowl with the sausage and onion mixture. Stir to combine.
To assemble the lasagna, pour the reserved 1/2 cup of marinara into the bottom of the skillet and spread around in an even layer. Place lasagna noodles on top in an even layer, breaking them to fit around the edges so that there aren't any large holes.
Pour half of the ricotta mixture on top of those noodles and spread out in an even layer. Top with 1/3 of the shredded mozzarella. Add another layer of noodles, then spread half of the meat/marinara mixture on top of those noodles. Add another layer of noodles, then spread out the remaining ricotta mixture and another 1/3 of the mozzarella. Add one last layer of noodles, then spread the meat/marinara mixture on top in an even layer. Top with the remaining 1/2 cup of parmesan and mozzarella in an even layer.
Move the skillet to the oven and bake for 45-55 minutes or until the sides are bubbling and the cheese is golden brown on top and around the edges.
