Prepare the onions: Bring a large pot of water to a boil. Trim both ends of the 6 large yellow onions, keeping the onions as intact as possible. Peel away the papery outer skins, then make an incision down one side of each onion, only cutting through to the center (don’t chop them in half). Add the onions to the boiling water and boil for 15 minutes, until softened and flexible enough to separate into layers. Drain and set aside until cool enough to handle.

Prepare the filling: Heat the olive oil in a large pan over medium heat. Add the onion and cook for 5 minutes, stirring often, until softened. Add the garlic, tomato paste, ground cumin, dried oregano, sea salt flakes, and freshly ground black pepper, to taste. Cook for 2 minutes, stirring often, until fragrant. Add the diced tomato, cooked brown lentils, arborio rice, walnuts, and vegetable broth, stir to combine, then cook covered for 5 minutes. Transfer to a large mixing bowl, and stir in the parsley and mint. Set aside.

Separate the onion layers: Gently peel away the larger outer layers, keeping them as whole as possible for stuffing. Set the smaller inner layers aside.

Make the tomato sauce: In a bowl, stir together the pureed tomato, vegetable broth, and olive oil.

Assemble the dish: Preheat the oven to 400°F (200°C). Choose a large oven-safe dish or pan with a lid (make sure it’s oven safe). Spread half of the tomato sauce over the bottom of the dish.

Stuff the onions: Place about 2 tablespoons of filling onto each onion layer (this can vary slightly depending on its size). Roll the onion around the filling, then place seam-side down in the prepared dish. Repeat with the remaining onion layers and filling, arranging the stuffed onions snugly in the dish.

Add the remaining sauce: Pour the remaining tomato sauce over the stuffed onions, making sure they are evenly coated.

Bake: Cover with the lid and bake for 30 minutes. Remove the lid and bake for another 30 minutes.

Serve: Top with a sprinkle of fresh parsley and serve straight away.