Season with salt and pepper (about ¼ teaspoon each).
Cook until onion and celery are beginning to soften, about 2-3 minutes, stirring regularly.
Remove the vegetables to a plate.
Add the noodles to the now empty inner pot.
Add enough water to cover the noodles by 1 inch.
Close and lock the lid.
Turn the pressure release valve to SEALING.
Press MANUAL high pressure and adjust the time to 1 minute.
Once the cooking time is up, turn the pressure release valve to VENTING to quickly release the pressure.
Carefully drain the noodles and return to inner pot.
Stir in the evaporated milk, seasonings, drained tuna, cheddar cheese (1 cup for milder cheese flavor and 2 cups for cheesier casserole), sour cream, and cooked vegetables.
Add the thawed peas, and stir in until combined.
Adjust salt and pepper to taste.
Serve topped with crushed crackers or chips and parsley, if desired.
Notes
Egg noodles: Be sure to grab the 12 ounce, not the 16 ounce bag, of egg noodles. Using the 16 ounce bag will give you drier results as there won't be as much sauce for the noodles.
Evaporated milk: You can use regular, low-fat, or fat-free evaporated milk.
Seasonings: Make sure you use garlic powder and onion powder, not salts. Otherwise, your dish will be much saltier.
Tuna: I like to use tuna in water. If you prefer tuna in oil, you can use that instead.
Sour cream: You can use regular or low-fat sour cream. I don't recommend using fat-free. If you're looking for a lighter substitute, plain, fat-free Greek yogurt works well.
Cheese: Feel free to use as much, or as little, shredded cheese as you like.
Topping: My family really likes crushed butter crackers (like Ritz) on top of this casserole. You can also use crushed potato chips, French onions, or any other topping that you like. I do recommend adding a topping for a little texture and bit of extra flavor.
Cooking time: I like to cook the noodles for 1 minute to keep them a little bit firmer. If you like softer noodles, increase the cooking time to 2 minutes.