In a large bowl, soak the lentils using one part dried lentils two parts well-salted boiling water and allow to sit for 15 minutes before draining.
In a large stockpot on medium heat, sauté the onions in the olive oil until translucent - about 10 minutes.
Add the garlic and cook another minute. Add the oregano, cumin, celery, and carrots and sauté for 5 more minutes.
Add the stock, tomatoes, tomato paste, parsley, and lentils. Cover and bring to a boil. Reduce the heat and simmer uncovered for 45 minutes - 1 hour, or until the lentils and vegetables are cooked through.
Check for seasoning and serve.
For a deeper flavor profile, when cooking the vegetables, allow them to brown and then scrape the bottom of the pan when adding the stock to distribute the flavor.
Lentil Soup often becomes quite thick during cooking or storing. If it gets a bit too thick simply thin it out with a little water or stock.