Heat neutral oil for deep-frying in a stock pot to 375° or until a cube of day-old bread browns in about 60 seconds. Slice one of the sausages diagonally and deep-fry for 2 to 3 minutes, or until brown and crisp. Add the basil leaves, frying for a few seconds until crisp. Remove the sausage and basil leaves using a slotted spoon and drain on a paper towel. Strain and reserve the oil into a small bowl or measuring cup.
In the same pot, heat olive oil. Add chopped red onion and cook until softened. Coarsely chop the remaining sausages and add them to the pan. Cook for about 5 minutes, stirring, or until the sausages are cooked. Stir in the lentils, tomatoes, and stock or water. Bring to a boil, then reduce the heat, cover, and simmer for about 20 minutes. Cool the soup slightly before puréeing it in the blender. Return the soup to the pot and reheat, adding seasoning to taste.
After ladling into individual bowls, sprinkle each serving with the deep-fried sausage slices and basil and swirl a little pesto through each portion just before serving. Enjoy with plenty of warm, crusty bread.