Boil the rotini until tender, about 8 minutes.
While the pasta cooks, preheat the oven to 350 degrees F.
Lightly butter two 1 1/2-cup baking dishes, or one 3-cup baking dish. Melt the 2 Tbsp. butter in a small pot over medium heat. Add the bell pepper, garlic and oregano and cook 2 minutes. Mix in the flour. Slowly whisk in the milk, bring to a simmer, and then remove from the heat. Stir in the cheese, salt and pepper.
Drain the cooked rotini and add with the tuna and green onion to the sauce and then spoon it all into the baking dish or dishes. Top with the bread or crackers crumbs and bake 20 to 25 minutes, or until golden.