In a large skillet, heat the olive oil over medium high. Once the oil is hot, add the onion. Cook the onion, stirring often, until it is very soft and beginning to nicely brown and break down, about 8 minutes. Add the carrots, celery, Italian seasoning, salt, smoked paprika, and pepper. Sauté until the carrots and celery are barely soft, about 2 minutes. Add the garlic and cook just until fragrant, about 30 seconds. Transfer to a 6-quart or larger slow cooker.

To the slow cooker, add the lentils, diced tomatoes, crushed tomatoes, and vegetable broth. Cover and cook on high for 3 to 4 hours or low for 6 to 8 hours, or until the lentils are tender and retain a slight amount of pleasant chew but are not mushy.

Remove the Parmesan rind (if using). Stir in the red wine vinegar and sugar. If the soup is too thick for your liking, thin with a bit of water or additional broth until you reach your desired consistency. Taste and adjust the seasonings as desired. Serve hot with a sprinkle of parsley, Parmesan (if using), and bread for dipping.