Boil pasta until just tender, about 8 to 10 minutes. While it cooks, place 3 Tbsp. of butter in a medium-sized pot set over medium heat. When melted, mix in flour, oregano and paprika and cook and stir 1 minute.
Very slowly, while whisking, mix in milk. Bring the sauce just to a simmer, stirring frequently, and then remove from heat. Mix in ¾ of cheese; season with salt and pepper. When pasta is cooked, drain well and mix with tuna and peas into the sauce.
Preheat the oven to 375°F. Lightly butter 4 small (about 1½ cup) baking dishes or foil pans. Divide the pasta between the baking dishes. Top with remaining cheese and sprinkle with panko. (The casseroles, at this point, could be cooled, wrapped, dated and frozen until ready to thaw and bake.)
Set casseroles on a baking sheet. Bake for 25 minutes, or until golden and bubbly. If desired, sprinkle each casserole with a little green onion.
Note: To warm milk, place in a glass measuring cup and microwave 1 to 2 minutes. Panko are coarse Japanese-style breadcrumbs sold in our Asian foods aisle.