To prepare your mise en place, finely dice the onion. Mince the garlic and chilies. Remove the curry leaves from the stem and set aside.
Set up a strainer over a bowl. Break open each whole tomato and remove the seeds, allowing the juice to collect in the bowl below. Place the de-seeded tomatoes into a separate bowl and roughly chop with kitchen scissors. Measure out the 2 cups of the strained tomato juice. Set aside.
Gather the stock, red lentils, cumin and oil and set aside.
To make the soup, heat a heavy-bottomed pot over medium-low heat and add the oil, followed by the onions, garlic and chilies. Sweat until the onions are translucent, about 8 to 10 minutes. Add the cumin and curry leaves and cook until fragrant, about 1 minute or so.
Add the chopped tomatoes, tomato juice, stock, salt and season with pepper to taste. Bring to a simmer.
Once simmering, add the lentils and continue to simmer until the lentils are cooked through, about 20 minutes or so.
Once the lentils are cooked through, season the soup with salt and pepper to taste.
For a thicker texture, you may want to partially blend the soup with an immersion blender.
Serve the soup in warmed bowls and garnish with chopped cilantro, freshly squeezed lemon and chili flakes.