Chop the zucchini and onion, mince the garlic, zest and juice the lemon, and grate the Parmesan. Heat the broth until hot.
Heat olive oil in a large sauté pan over medium heat. Cook the onion 3–4 minutes until softened. Add garlic and cook 30 seconds. Add diced zucchini and a pinch of salt, sauté 3–5 minutes until just tender. Remove half the zucchini and set aside.
Push vegetables to the side, add the orzo to the pan and toast 1–2 minutes. Pour in hot broth, bring to a simmer, and cook uncovered 8–10 minutes, stirring occasionally, until orzo is al dente and liquid is mostly absorbed. Add more broth if needed.
Return reserved zucchini to the pan. Stir in butter, Parmesan, lemon zest, and lemon juice. Season with salt, pepper, and red pepper flakes if using. Garnish with fresh herbs and extra Parmesan before serving.
Notes
Make-ahead: Cook components separately and combine when reheating to preserve texture.
Storage: Refrigerate up to 3 days in an airtight container.
Substitutions: Use gluten-free orzo for a gluten-free version; add spinach or peas in the last minute of cooking.