Preheat the oven to 375F. Bring a large pot of water to a boil.
Trim the top and bottom off each onion and peel away the papery skin, removing the outer 1–2 tough layers. Using a small knife, carefully slice down one side of each onion from top to bottom, cutting just to the center without slicing all the way through.
Boil the onions whole for 10 to 12 minutes until softened, then drain and let cool slightly.
For the filling, heat two tablespoons of olive oil in a skillet over medium heat. Add the red onion and garlic and sauté for 5 minutes until translucent. Stir in the tomato paste, salt, oregano and dried basil.
Add the lentils, rice, tomato, parsley and vegetable broth. Stir to combine and heat through, then remove from heat.
In a separate bowl or jug, prepare the tomato sauce. Whisk together the crushed tomatoes, vegetable broth, and a drizzle of olive oil.
Once onions are cool enough to handle, separate them into layers.
Spoon the lentil mixture into the onion cups and arrange in a cast iron pan or baking dish. Pour the tomato sauce around the stuffed onions, covering the base of the dish.
Cover tightly with foil and bake for 40 minutes, then uncover and bake for another 15-20 minutes until the onions are golden and tender and the sauce has thickened.