In a large pot over medium heat, add the olive oil, diced onion and garlic. Stir and saute for 2-3 minutes or until fragrant.
Add the celery and carrot to the pot and saute for another 3-4 minutes or until they begin to soften.
Add the orzo and stir for 1-2 minutes to lightly toast the orzo. Pour in the vegetable broth, stir, put the lid on the pot and bring to a boil. Reduce the heat and simmer for 10-15 minutes or until the orzo is cooked.
Follow this step ONLY if you do not plan to freeze this soup OR when you are thawing it out. While the soup is cooking, prepare the cornstarch slurry. In a bowl or glass measuring cup, whisk together the cornstarch and the water until it's combined. Set aside.
Add the lentils, navy beans, juice from 1 lemon, oregano, salt, pepper and chili flakes and stir well. You may want to add an additional 1-2 cups of water if you prefer a more watery soup.
Add the cornstarch slurry and mix well. Adjust the seasonings to your preference and serve. Enjoy!