Place dried lentils in a large bowl and cover with hot water, let soak for 30 minutes while you prep the remaining ingredients, you can avoid this step if you are using canned lentils.
Preheat oven to 350F.
Peel onions and cut off bottom and tops. With a knife make a slice along the side right through to the centre. Carefully peel each, layer ensuring you keep the onion layers intact, about 5 layers from each onion..
Finely chop the leftover onion from the centre and set aside.
Drain lentils and place back in bowl. Add chopped onions, rice, parsley, dried herb you are using, tomato puree 2 tbsp olive oil, 1/2 of lemon juice, salt and pepper, mix well. Stuff onions with lentil mixture ensuring you seal the cut side by overlaying the the open sides of onion. Place them standing up in a baking dish large enough for them to fit in snug, so the onions stay closed. I used an 9' inch round dish with lid.
Drizzle onions with remaining 2 tbsp of oil, remaining lemon juice. Pour in the vegetable stock on the sides. Cover and cook for 40-45 mins. Remove lid and broil until slightly browned, about 2-3 minutes. Place lid back on, turn off oven and let stand covered in oven for 15-20 minutes or until they have absorbed their liquid.