Cook egg noodles according to package directions until al dente and drain.

Coat the interior of a slow cooker with nonstick cooking spray.

In a large bowl, combine condensed cream of mushroom soup, milk, and 3/4 cup shredded cheese; stir until smooth.

Mix in tuna, onion, peas, salt, and pepper.

Incorporate the cooked noodles into the mixture and stir gently.

Pour the mixture into the prepared slow cooker.

Scatter the remaining shredded cheddar cheese over the top.

Cover; cook the casserole on low for 3-5 hours until heated through and cheese is melted.