Melt the butter in a large broad saucepan or deep skillet. Add the scallions and cook, stirring until wilted, about 2 minutes. Add the heavy cream and heat to boiling. Boil until slightly thickened and reduced to about 3/4 cup, about 5 minutes. Season with salt and pepper. Cook the gemelli in boiling salted water until firm to the bite, according to package directions. Remove from the heat and add 1 cup cold water to stop cooking; drain. Add to the heavy cream the shrimp and peas; stir until heated through over low heat.