Heat a large Dutch oven or soup pot over medium-high heat. Add the olive oil, then add the onion, carrot, celery, and leek. Sauté until softened, 4-5 minutes. If it starts to brown quickly, turn down the heat.

Add the chopped rainbow chard stems and sauté for 1 minute. Add the tomato paste, stir, and cook for 2-3 minutes until it turns a bright brick red color (add another tablespoon oil if it looks dry). Turn the heat to medium and add the garlic, cooking for 1 minute more until very fragrant.

Add the thyme, bay leaf, drained & rinsed beans, salt, pepper, and vegetable broth or water. Add the large, whole dark green end of the leek if you have it. Bring to a low boil, then reduce to a simmer for 30 minutes to let the aromatics flavor the soup.

Remove the leek stem, bay leaf, and thyme stems if they are easy to fish out. Bring the soup to a rolling simmer, add the orzo pasta, and cook according to the pasta package instructions (usually about 10 minutes).

Taste and adjust seasoning. Serve in soup bowls with a squeeze of lemon, and optionally freshly grated Parmesan cheese.