In a 3 quart saucepan, heat oil until smoking. Add onions, sliced garlic and sage. Cook until softened but not brown, about 6 to 8 minutes. Add lentils and chicken stock. Add water to 1 inch over lentils and bring to a boil. Lower heat and simmer for 25 to 30 minutes, keeping the level of the liquid just over the lentils by periodically adding water. Season with salt and pepper to taste.