1. Heat oil in a large, non-stick frying pan. Sauté diced onion until it starts to caramelize.
2. Add mushrooms and sauté for a few minutes longer, until they begin to shrink a bit.
3. Add chopped meat. While it’s browning, use a potato masher to mash the mixture into small pieces. Continue to stir until meat is mostly brown, with just a bit of pink still showing.
4. Add remaining ingredients and cover the pan. Reduce heat and cook for about 10 minutes, stirring and checking occasionally to make sure that it’s not burning.
5. Fill taco shells with meat mixture. Sprinkle each one with some of the French-fried onions. Serve hot.