Grease the slow cooker well. Stir in the crushed tomatoes, garlic, worcestershire sauce, sugar and seasonings. Set this aside.
In a large skillet, heat up the olive oil over medium-high heat. Slice the sausages into thirds, then place them into the pan and brown for just a couple of minutes on each side, until golden brown. You don't have to worry about them being fully cooked through at this point, as they'll finish cooking in the slow cooker.
Once browned all over, transfer the sausages into the slow cooker. Slice the onion in half, then slice it so that you get thin, half moon shaped slices. Slice the bell peppers into strips. Add the onions and peppers to the slow cooker and stir everything together.
Cook on low heat for 5-6 hours. The sausage, onions, and peppers should be very soft and tender. Serve immediately however you'd like.
