Boil the pasta: Bring a large pot of salted water to a boil. Cook the noodles until just shy of al dente (1 minute less than package). Drain and set aside; toss with a drizzle of oil to prevent sticking.
Start the roux: In a large deep skillet, melt the butter over medium heat. Whisk in the flour and cook 1 minute until it smells nutty. Do not brown.
Build the sauce: Slowly whisk in the broth, then the milk, until smooth. Add onion powder, garlic powder, parsley, paprika, salt, pepper, and Dijon. Simmer 3–4 minutes, whisking, until the sauce thickens enough to coat a spoon.
Cheese it up: Reduce heat to low. Stir in the cheddar a handful at a time until melted and silky.
Add tuna and peas: Gently fold in the drained tuna and frozen peas. Warm through 2 minutes. Avoid aggressive stirring so the tuna stays in nice flakes.
Combine with pasta: Add the cooked noodles to the skillet and toss to coat. If the sauce feels too thick, splash in a bit of milk or hot pasta water to loosen.
Taste and finish: Adjust salt and pepper. Squeeze a little lemon over the top for brightness. Garnish with herbs or a shower of crushed crackers if you like extra texture.
