PREP: Open the tuna and drain thoroughly. Fluff with a fork and set aside.
ADD EVERYTHING TO A SKILLET: In a large 10-inch nonstick skillet (3 quart; 2.8-liter), pour in 2 cups of uncooked pasta; the drained tuna; milk, water, butter, and seasonings (dry mustard powder, paprika, garlic powder). Add salt and pepper to taste; I add 1 teaspoon fine sea salt and 1/4 teaspoon pepper.
BOIL: Bring to a boil over high heat, stir, and then cover with the lid. Reduce the heat to low or just a few notches above low.
SIMMER: Allow to simmer until the pasta is completely cooked through and the liquid has evaporated (about 10-15 minutes depending on your stovetop heat; check at 10 minutes). During the cooking time, stir the ingredients every 5-7 minutes to keep the pasta from sticking to the bottom. While pasta is simmering, grate the cheese.
ADD CHEESE AND PEAS: Once the liquid is absorbed, remove the skillet from the heat. Stir in the grated Cheddar cheese until melted. Once melted, add in the frozen peas and stir to warm through.
ENJOY: Taste and adjust for any additional seasonings needed. Add salt and pepper to taste as well as any other additional seasonings (more garlic powder, paprika, etc.). Enjoy while hot. If desired, garnish individual plates and freshly chopped parsley and/or a drizzle of hot sauce.