Heat oil in a 5- to 6-qt. pan over medium heat. Add garlic and cook until fragrant, 30 seconds. Reduce heat to low and add brussels sprout leaves and cores.
Cook, stirring often, until brussels sprouts soften and start to brown, about 10 minutes. Add 2 tbsp. water, cover, and cook until cores are tender, about 5 minutes more. Season with salt and zest.
Note: Nutritional analysis is per serving.