The sure-fire combination of sausages, peppers and onions is ready-when-you-are when you make them in the slow cooker. (Once the sausages are cooked through and the peppers and onions are tender, they can hold on “warm” for up to 2 hours.) Searing the sausages in a skillet is the only cooking you’ll have to do, but it’s an important step that fortifies the sauce with the sausage’s brawny, spiced juices. The sweet peppers and onions will tangle in the light, tangy tomato sauce. (Save extra…