Bring a large pot of salted water to a rolling boil. Add the 12 ounces of egg noodles and cook according to the package directions, stopping when they are slightly underdone and still pleasantly firm. Drain the noodles thoroughly and reserve them. Avoid cooking them completely at this stage, as they will continue softening in the casserole.

In a large mixing bowl, whisk the condensed cream of mushroom soup with 2 cups of chicken broth until the mixture becomes smooth and evenly blended. Add the drained tuna, diced onion, Italian seasoning, salt, and black pepper, stirring until the ingredients are distributed throughout the sauce. Carefully fold in the partially cooked egg noodles until they are uniformly coated.

Transfer the prepared noodle mixture to the slow cooker and cover with the lid. Cook on LOW for 3 to 4 hours or on HIGH for 1½ to 2 hours, stirring occasionally when practical, until the casserole is thoroughly heated and the flavors have combined.

Approximately 15 minutes before the cooking period is complete, remove the lid and gently stir in the drained sliced mushrooms and frozen peas. Add ½ cup of the shredded cheddar cheese and fold it evenly through the casserole. Scatter the remaining ½ cup of cheddar over the surface, replace the lid, and continue cooking for the final 15 minutes, allowing the cheese to melt.

Once the cheese has melted and the casserole is piping hot, sprinkle up to ¼ cup of dry breadcrumbs over the surface, if using. Serve the casserole hot.