In a large skillet, over medium heat, cook beef, hot sausage and onion, stirring until meats are no longer pink. Drain.
In a small bowl, combine ricotta cheese, Parmesan cheese, salt, pepper and garlic powder. In another bowl, combine pasta sauce and water.
Coat a slow cooker (I use a cooking bag and spray it) Arrange 4 uncooked noodles in slow cooker, breaking noodles to fit. Spoon 1/2 the meat mixture over he noodles, top with 1/2 the sauce mixture, then 1/2 the mushrooms. Spread the ricotta mixture over the mushrooms and sprinkle with 1/2 the mozzarella cheese. Layer the remaining noodles, meat mixture, sauce, mushrooms and mozzerella cheese.
Cover and cook on LOW for 4 hours or until noodles are done. Let stand 10 minutes before lifting bag out of crock pot and placing it into a 9x13 pan. You may also keep the lasagna in the crock portion and serve and store from there.
Normally, most slow cookers are very forgiving if allowed to cook beyond the time specified but lasagna is the exception. If you cook longer than 4 hours, the cheese and noodles may scorch.