Preheat oven to 400 degrees. Lightly oil two 8-inch square (or other shallow 2-quart) baking dishes. In a large pot of boiling salted water, cook noodles until 2 minutes short of al dente; drain, and return to pot.
Meanwhile, in a 5-quart Dutch oven or heavy pot, heat oil over medium. Add bell peppers; season with salt and pepper. Cook until crisp-tender, 4 to 6 minutes. Add flour and cook, stirring, 1 minute. Gradually add milk, stirring until smooth. Cook, stirring occasionally, until mixture comes to a simmer.
Remove from heat; add mixture to noodles in pot, along with tuna, artichoke hearts, and scallions. Season with salt and pepper, and toss. Divide between prepared baking dishes, and sprinkle with Parmesan. Bake one, freeze the other, for later. Bake until golden and bubbling, about 20 minutes.