Fit food mill directly onto pan. Add tomatoes with juices and puree through mill into pan, leaving only seeds behind and scraping all tomato pulp from underside of food mill into pan; or puree tomatoes with juices in processor, then strain out seeds and add puree to pan. Add crushed red pepper. Simmer very gently over low heat until sauce thickens and reduces to scant 2 cups, stirring sauce and turning sausages occasionally, about 1 hour 15 minutes. Add slivered basil and simmer sauce 5 minutes longer. Using tongs, transfer sausages to plate and cool. Season sauce to taste with salt and pepper.