Cook the elbow macaroni according to package instructions until al dente. Drain and rinse with cold water to stop cooking. Set aside.
In a large mixing bowl, combine the condensed cream of mushroom soup, mayonnaise, and milk. Stir until smooth.
Add the shredded cheddar cheese, drained tuna, thawed peas, chopped onion, garlic powder, black pepper, and dried parsley flakes to the bowl. Mix well to combine all ingredients evenly.
Add the cooked and cooled macaroni to the mixture. Gently fold until the pasta is fully coated with the sauce and all ingredients are well distributed.
Transfer the mixture to a serving dish or individual bowls. The casserole is ready to serve immediately or can be chilled in the refrigerator for 30 minutes to 1 hour for a cooler dish.