How to Make It
Preheat the oven to 450°. Cook the farfalle pasta in a large pot of boiling salted water until al dente. Drain.
Meanwhile, in a large saucepan, melt 3 tablespoons of the butter. Add the onion and cook over high heat, stirring, until softened, about 3 minutes. Add the flour and cook, stirring, for 1 minute. Add the milk and bring to a boil. Cook the sauce over moderate heat, stirring occasionally, until thickened, about 3 minutes.
Add the farfalle pasta, frozen baby peas, sliced piquillo peppers, Parmigiano cheese and tuna and season with salt and pepper. Transfer the mixture to a large baking dish, a cazuela (casserole dish) or 4 individual gratin dishes.
In a small skillet, melt the remaining 1 tablespoon of butter. Add the panko and cook over moderate heat, stirring, until golden, about 1 minute. Sprinkle the panko over the casserole and bake for 10 minutes (5 minutes for individual gratins), or until bubbling. Serve right away.