Remove coarse outer leaves from cabbage, quarter and core. Cut into fine fine shred, either by hand or with food processor. Set aside. Peel onions; cut in half lengthwise. Cut crosswise into thin half rings; set aside.
In large wok or large wide pot over medium heat, heat oil. When very hot, add the cumin and the fennel seeds. As soon as the seeds turn a shade darker (just a few minutes) toss in the sesame seeds. Stir for a second before adding onions. Stir fry 2 or 3 minutes or 'til browned on the edges. Cover tightly and turn heat to low. Cook 2-3 minutes or 'til cabbage wilts. Uncover and cook over high heat, stir fry 5 minutes or 'til vegetables are very tender.