Toast panko in a 4- to 5-qt. saucepan or Dutch oven over medium heat about 3 minutes, stirring occasionally. Transfer quickly to a small bowl; stir in parsley and lemon zest and set aside.
Heat oil in same pan over medium heat. Add onion and cook, stirring occasionally, about 4 minutes or until tender. Whisk in flour and cook 1 minute, stirring constantly. Whisk in broth and bring to a boil, stirring occasionally.
Stir in cauliflower; cover and simmer 7 to 8 minutes or until tender, stirring halfway through. Stir in Parmesan and season to taste with salt and pepper. Sprinkle with panko mixture and serve.