In a small pan, heat the olive oil until moderately hot. Add the garlic, ginger, five-spice powder, and coriander. Cook for 2 minutes, stirring to prevent burning. Add 1/2 cup water, the rice wine, soy sauce, orange juice, and zest. Bring to a boil, reduce the heat, and simmer for 5 minutes. Remove from the heat and add the sesame oil.
Arrange the portabellas in a shallow container in one layer and pour the hot mixture over them. Marinate for 1 to 2 hours, turning the mushrooms occasionally. Remove the portabellas and let them drain for a few minutes; reserve the marinade.
Heat the vegetable oil in a large sauté pan until hot. Sear the portabellas in one layer on one side, undisturbed, for about 5 minutes. Turn and sear the other side. Lower the heat if the portabellas are browning too fast. When tender, transfer them to paper towels to absorb some of the grease. Slice each portabella very thinly on the diagonal as you would a flank steak; drizzle with some of the reserved marinade, if you like.