Discard cabbage outer leaves and cut into quarters. Cut the stem off of each quarter, and cut then cut it into big square chunks.
In a large pot over medium-high heat, add the olive oil until it shimmers. Add the onions and leeks. Stir well for 2-3 minutes until softened and translucent. Add carrots, celery leaves, and tomato paste for another 2-3 minutes. Finally the cabbage for another 5 minutes. Stir well and season to taste.
Pour carefully hot water (or vegetable broth) to cover all the ingredients and reduce the heat to a simmer. Cover with a lid. Stir occasionally, until the cabbage is soft for about 10 minutes. Don’t overcook it. Cooking time depends mostly on the size of cabbage chunks.
Meanwhile, prepare the rice and thoroughly rinse it under cool water. Drain. When cabbage is soft, add the rice in the pot. Taste and season if needed, and simmer for about 10-15 minutes until rice is ready.
If you want more liquid in your stew, uncover the pot while simmering the rice, add some hot water, about ¼ cup (60ml). Repeat if needed but don’t add too much. When the rice is ready, remove from heat.
If you like thicker stews, remove from heat and keep the pot covered until the liquid is absorbed.
Serve with fresh lemon juice, freshly ground pepper, parsley, and feta cheese (for vegetarians).