Wash the spinach well and remove the stems. Tear the leaves into pieces the size of a silver dollar. Dry and reserve in the refrigerator.
Heat 1/4 cup of the olive oil in a skillet. Add the onion rings and cook over medium heat until they appear translucent, about 4 minutes. Add the mushrooms; saute for 5 minutes. Add the garlic and saute for 1 minute. Stir in the remaining oil, vinegar, juice and seasonings; remove from heat. Toss with spinach and honey tangerines.