Bring a large pot of salted water to a boil. Add egg noodles and cook for 1-2 minutes less than package directions indicate (they'll finish cooking in the sauce). They should be just shy of al dente. Drain well but do not rinse.
While the noodles are cooking, melt butter in a large skillet or deep sauté pan over medium heat. Add the diced onion and celery, cooking until softened but not browned, about 3-4 minutes.
Reduce heat to medium-low and add the condensed soup and milk to the skillet, stirring to combine with the sautéed vegetables. Allow the mixture to warm through for about 2 minutes, then stir in the sour cream until fully incorporated and smooth.
Gently fold in the drained tuna, frozen peas, garlic powder, and black pepper. The residual heat will warm the tuna and cook the peas to perfect tenderness. Stir in the drained noodles, ensuring they're evenly coated with the creamy sauce. Cook for 2-3 minutes until everything is heated through.
Sprinkle the shredded cheese over the mixture and stir just until melted. Remove from heat and top with fresh parsley. If using the optional topping, sprinkle crushed potato chips or panko breadcrumbs over individual portions for a satisfying crunch.