Scrub and halve or quarter the new potatoes. Peel and finely chop the cloves from the garlic bulbs. Juice half of the limes, and cut the remainder into wedges. Deseed and finely chop the red chillis.
- 1 To make the mojo de ajo, put the oil in a small, heavy-based saucepan and heat gently. Add the garlic and cook very gently for 20 minutes, until soft and caramelised - don't let it get too brown or it will taste bitter.
- 2 Add the chillies and cook for a further 5 minutes. Move on to step 4 while you wait, then move to step 3 when the chillies are ready.
- 3 Remove from the heat and stir in the lime juice; season and set aside.
- 4 Meanwhile, parboil the potatoes in boiling salted water for 5-10 minutes, until just tender. Drain and refresh under cold running water.
- 5 Thread the potatoes and prawns alternately onto metal skewers, so that there are 2 skewers per serving.
- 6 Pour half the mojo de ajo into a ramekin and use to brush the potatoes and prawns.
- 7 Griddle or barbecue the skewers for 2-3 minutes each side, until the prawns are pink and cooked through.
- 8 Mix the remaining mojo de ajo with the mayonnaise.
- 9 Serve the skewers with the mojo de ajo mayonnaise and lime wedges.