If you have a slow cooker that tends to burn food on the outside edge, line your slow cooker with an approved liner.
Grease the bottom and sides of the slow cooker crock with butter if you're not using a liner.
In a large skillet, melt the butter. Add in the diced onion and saute over medium heat until the onions are tender-crisp. Add in the mushrooms and continue to saute until the mushrooms have reduced in size and are soft. Add in the garlic and saute until browned and fragrant, around two minutes. Remove from the heat and set aside.
Combine the Alfredo sauce and the water in a large measuring cup.
Combine the ricotta cheese, eggs, oregano, salt, and pepper in a bowl.
Pour half of one jar of Alfredo sauce into the bottom of the crockpot. For your bottom layer, place 3-4 noodles on top of the sauce. You will have to break them up and place them patchwork, but this is okay. You want to create a solid layer of pasta.
Place 1/3 of your ricotta mixture on top of the noodles, then 1⅓ cups of chicken in an even layer. Place 1/3 of the onions and mushroom mixture on top of the chicken in an even layer. Sprinkle 1 cup of shredded mozzarella on top.
Divide the remaining sauce into three portions.
Create another layer of noodles. Pour one portion of the Alfredo sauce on top. Repeat with another 1/3 of the ricotta, 1 ⅓ cups of the chicken, ⅓ of the mushrooms and onions. Top with another 1 cup of mozzarella cheese.
Repeat again. Top with a layer of noodles again. Pour one portion of the Alfredo sauce on top. Use the last of the ricotta mixture, the remaining chicken, and the remaining mushrooms and onions. Top with another 1 cup of mozzarella cheese.
Top with a layer of noodles again.
Pour the last portion of sauce on top, then top with 2-3 cups of mozzarella cheese. Don't be shy with the cheese at this point. create a nice thick layer.
Place the lid on top of your slow cooker and cook on low for 5-6 hours. This shouldn't be cooked longer than 6 hours. Slice in the slow cooker and then remove portions to plate.
Notes
The amount of noodles it will take depends on the size of slow cooker you are using. Make sure they are no-boil, or this recipe won't work.