Prick sausages all over with fork (to keep them from bursting).
Melt butter in large skillet over medium heat. Add sausages and cook until browned on all sides. Transfer to plate and set aside.
Add onions or leeks, shallots and garlic to skillet and cook, stirring, until softened, about 5 minutes.
Scrape onion mixture into slow cooker. Add potatoes, 1/2 cup of chicken broth, green peppers, tomatoes, lentils, vinegar and brown sugar. Add 1 teaspoon of rosemary and reserved sausages.
Cover slow cooker and cook on low setting for 8 to 10 hours or until stew is thick, adding more of remaining broth if needed toward end.
Season with salt and pepper. If stew is too acidic, add small pinch of additional brown sugar. Serve topped with cheese and sprinkled with remaining rosemary.