How to Make It
In a medium saucepan of boiling salted water, cook the beans until tender, about 4 minutes. Drain and refresh with cold water. Cut the beans into 2-inch lengths.
In a large skillet, melt 1 tablespoon of the butter in the olive oil. Add the red pepper and the white of the scallion and toss over high heat for 1 minute. Add 1/2 cup of water, reduce the heat to moderately low, cover and cook until the pepper is tender, about 4 minutes. Add the Romano beans and cook until warmed through. Stir in the sour cream and scallion greens and remove from the heat.
Meanwhile, in a large saucepan of boiling salted water, cook the pappardelle, stirring, until al dente, about 3 minutes. Drain the noodles and transfer to a platter. Toss with the remaining 1 tablespoon butter. Season the vegetable mixture with salt and pepper, spoon over the pasta and garnish with the basil.