Lightly grease the interior of a large slow cooker.
In a large pan, drizzle olive oil over medium heat. Crumble in the ground beef and sausage and cook, breaking up the pieces with a wooden spoon, until browned. Stir in the onion and garlic. Continue to cook until the onions have softened. Discard any excess grease.
Stir the marinara sauce and water into the cooked meat. Season with salt and pepper and remove from heat.
In a medium bowl, combine the ricotta cheese, parsley, and salt and pepper to taste.
Cover the bottom of the greased slow cooker with 2 cups of the meat sauce. Top with a layer of lasagne noodles, breaking them as needed to fit the entire surface. Cover with 1/3rd of the ricotta mixture. Sprinkle with mozzarella cheese and parmesan. Repeat the layers again: sauce, noodles, ricotta, mozzarella, and parmesan. Repeat the layers a third time, then for the final layer top the cheese with one last layer of noodles and cover with about 1 1/2 cups of the meat sauce.
Set the slow cooker to high and cook for 3-4 hours, until the mixture is bubbly and the noodles have cooked through. The lasagna can also cook on low for 6-8 hours. At the end of cooking, add a last sprinkling of mozzarella and parmesan, cover, and cook for another 10 minutes to melt the cheese.
Remove from heat and allow the lasagna to rest 15-20 minutes before slicing and serving.