Using a Y-peeler, remove two 1"-thick strips of zest from the lemon. Add onion and lemon zest to the butter and cook, stirring often, until softened, about 5 minutes. Add garlic and cook, stirring, until fragrant, about 1 minute. Stir in rice, broth, and 1 teaspoon salt. Bring to a simmer, cover, and transfer to the oven.