Steam broccoli and green beans for 2 minutes. Heat the oil in a wok until very hot. Add mustard seeds. Cook until they begin to 'pop'.
Add onion and garlic. Sauté for 3 minutes. Add potatoes, sweet corn, celery, capsicum, broccoli and beans. Reduce heat and stir-fry for 5 minutes. Stir in garam marsala and cayenne pepper. Serve immediately.