Ingredients :

• Green or brown lentils — Stick to green or brown, and definitely not red. Red lentils completely disintegrate during slow cooking — you’ll end up with a uniform mush with no texture. Green ones hold up, keeping a slight bite, which gives the soup its heartiness.
• Russet (or sweet) potato — The classic Russet is the secret ingredient here. It breaks down while cooking and naturally thickens the broth. Sweet potato adds a slight sweetness that contrasts well with the vinegar’s acidity. Dice it into pieces smaller than 2 cm — the smaller they are, the more they melt into the soup.
• Red wine vinegar — A teaspoon might seem like nothing. But it’s what keeps the soup from being bland. The acidity wakes up every flavor without being tasted directly. If you don’t have any, apple cider vinegar works great. Only add it at the end of cooking with the tomatoes — never at the beginning.
• Diced tomatoes — A can of fire-roasted tomatoes gives more depth than an ordinary can — the slight smoky note fits in well. Regardless of the brand, the key is adding them only at the end. The acidity in tomatoes slows down the softening of lentils if added too early.