Heat a large skillet over medium-high heat. Add the butter and garlic and cook until fragrant, about 1 minute.
Add the dry orzo and fresh thyme to the skillet and toast, stirring, until the orzo is golden, about 2–3 minutes.
Stir in the grated zucchini and shredded kale and cook until wilted, about 2–3 minutes.
Pour in 1/2 cup of the broth to deglaze the pan, scraping up any browned bits. Add the lemon zest, lemon juice, and onion powder, then season with salt and pepper.
Slowly add the remaining 2 1/2 cups broth and bring the mixture to a boil over high heat. Reduce to a simmer and cook, stirring often, until the orzo is al dente and the mixture is very creamy, about 8–10 minutes.
Stir in the milk, grated Parmesan, and chopped parsley and cook a few minutes more until warmed and creamy. If the mixture becomes too thick, loosen it with additional broth to reach desired creaminess.
Divide among bowls and top with extra thyme, butter, or Parmesan if desired before serving.