In a large skillet, heat half (1½ tbsp) of the vegetable oil over medium heat. Add the sliced onion and cook, stirring occasionally, for 10 to 12 minutes, until the onions are a rich brown. While the onions cook, heat the beans in a small saucepan.
Stir the taco seasoning mix and water into the onions, and cook for 1 to 2 minutes more to thicken. Spoon the onions into a serving bowl and set aside.
In the same skillet, increase the heat to medium-high and add the remaining oil. Cook the garlic in the hot oil for 10 seconds, just until it's fragrant, then add the spinach leaves, a handful at a time, stirring and adding more as they wilt. Sprinkle with salt and diced green chiles and continue to cook until the spinach is tender, 3 to 4 more minutes.
To serve, mound beans, spinach and onion in taco shells or Fiesta Flats(tm) and top with crumbled feta and taco sauce.