Place the garlic in a non-metallic dish with the lime rind and juice, cumin, coriander and paprika. Stir in half the olive oil. Fold in the beef strips and then cover with clingfilm and marinade for up to 2 hours (or overnight is best).
To make the fajitas, heat the remaining olive oil in a large frying pan and gently fry the onion and peppers for 6-8 minutes over a low heat until softened, stirring occasionally. Remove with a slotted spoon to a bowl and keep warm.
Meanwhile, make the salsa, place the tomato in a bowl with the avocado, spring onions, chilli and mix well to combine. Season lightly and set aside at room temperature to allow the flavours to combine.
Reheat the frying pan; add the marinated beef strips and sauté for 4-5 minutes until cooked though and lightly golden. Return the onion and pepper mixture to the pan and stir-fry for another 2-3 minutes until well combined and heated through. Season to taste.
Heat a frying or griddle pan. Add a soft flour tortilla and heat for 30 seconds, turning once until soft and pliable. Repeat with the remaining tortillas and stack them up on a warmed plate. Transfer the sizzling beef mixture into a warmed bowl or platter and garnish with the fresh coriander. Hand around the warmed tortillas, salsa and soured, allowing each person to assemble the fajitas themselves.