Preheat oven to 425 F and line a rimmed baking sheet with tin foil.
Open the cans of tomatoes and drain them well, reserving the juice. Taking the whole tomatoes, give them a little squeeze to slice them open and let the juice drain out of them as well. Then take the whole tomatoes and place them on the baking sheet in a single layer, spreading them out so they have room to roast. Drizzle olive oil on them and then sprinkle with the brown sugar, some salt and pepper.
Place the pan in the oven and roast for 30 minutes. Add the garlic cloves and roast for another 10-20 minutes, until the garlic cloves are soft. Remove from the oven and set aside.
In a big soup pot, sauté your chopped onion in oil until it's soft and translucent. Add in the tomato paste, reserved tomato juice, stock, sun-dried tomatoes, herbs, and lentils. Using a spoon or fork, smash up the roasted tomatoes and garlic into smaller pieces before adding them to the pot as well. Bring to a simmer for about 15 min before adding the oats, and simmer for about 10-15 min more until everything is tender and cooked through.
Serve and enjoy!