Bring a large pot of salted water to a boil over high heat. Add the egg noodles and cook per package directions. A minute or so before the noodles are done, add the frozen peas to the boiling water. Once the noodles are cooked to your liking, drain the noodles and peas and set aside.
While the noodles are cooking, set another large pot, pan, or skillet over medium heat. Add the butter and swirl until melted. Stir in the flour and continuously whisk for 2 minutes. Continue to whisk vigorously as you slowly and gradually pour in the milk. Stir until smooth and bring to a simmer, whisking regularly to prevent the bottom from burning. Cook for a minute or two until the sauce has thickened. Remove from the heat, stir in the salt, garlic powder, and pepper. Gradually blend in the cheddar until melted, and then stir in the drained tuna and the parsley, if using.
Stir the drained noodles and peas into the sauce and mix until everything is well coated. If desired, combine the cracker crumbs and the Parmesan in a small bowl and evenly sprinkle the mixture over the entire skillet. Serve immediately.
The way the recipe is written is the fastest way to make it. However, if you don't mind it taking a little longer and you'd prefer to conserve on dirty dishes, you can make everything in the same pot. Start out by boiling the noodles and peas (as directed) in a large pot or Dutch oven. Drain them into a colander, rinse with hot water, and set aside. Then proceed with the recipe by melting the butter in the same pasta pot and making the white sauce (as directed). Finally, add the noodles and peas back to the pot at the very end.