Enjoy half of this sweet and spicy meat sauce tonight over spaghetti, and the rest later—it will keep frozen up to six months.
Heat oil in a 4- to 5-quart saucepan over medium-high heat. When oil shimmers, add sausages. Brown sausages, breaking them up into small pieces as they cook, just until sausages are cooked through, 5 minutes.
Add tomato paste, onion, carrot, and garlic and continue cooking, stirring occasionally, until vegetables begin to soften, about 3 minutes.
Stir in tomatoes and rosemary and break up tomatoes into pieces with the side of a spoon or scissors. Bring to a boil. Reduce heat, partially cover, and simmer, stirring occasionally, until sauce thickens and reduces slightly.
Cool meat sauce to room temperature, then transfer to freezer-safe containers with tight-fitting lids and freeze until ready to use, up to 6 months.