Grate the zest from the oranges. Using a sharp knife, remove the orange rind and all of the white skin. Then, holding each orange in one hand over a bowl to catch the juices, cut out the segments. Squeeze all the juice out of the left over orange.
In a shallow non-metallic dish, stir together the taco spice mix, with the oil and orange zest and juice (keeping a couple of tablespoons of the juice for later). Add the lamb fillets and turn to coat in the mixture. If you have time, leave to marinate for 2 to 3 hours.
Preheat a griddle pan or barbecue to moderately hot and cook the lamb fillets for 5 to 7 minutes for medium and 7 to 10 minutes for well done, turning occasionally and basting with any remaining marinade so that they’re all lovely and coated. Leave the lamb to rest for a couple of minutes, then slice into thin strips.
Throw the beans, red onion, parsley and spinach in a bowl with the orange segments and the rest of the orange juice. Mix gently.
Warm the flat bottom taco shells, following the instructions on the box.