
Heat the olive oil in a large heavy-bottomed pot over medium heat. Add the onions, carrots, and celery and cook until soft and translucent, 5 minutes. Add the garlic, oregano, salt, and pepper, and cook until fragrant, 1–2 minutes.
Stir in the zucchini, rice, and bay leaves until well incorporated, about 1 minute more. Add the vegetable broth and water, and bring the mixture to a boil.
Reduce the heat to a low simmer and cook, covered, until the rice is fluffy and the vegetables are soft, about 30 minutes.
Remove from the heat and stir in as much of the lemon juice as you'd like. Garnish with chopped parsley and lemon slices, if desired. Serve immediately.