In a medium pan, combine the sugar, juice of 1 lime and 3 tbsp water. Cook over a low-medium heat, stirring until the sugar dissolves. Increase the heat to high, then continue to cook, swirling the pan carefully so the sugar cooks evenly, until the mixture becomes a deep amber caramel. Once the caramel is nice and dark, remove from the heat and immediately add 60ml water (be careful, it will splutter and bubble up). When the bubbling subsides, stir in the fish sauce, then set aside to cool a little.
Heat the oven to 140°C fan/gas 3. Put the rib racks in a roasting tin. Mix half the fish sauce caramel with the soy sauce, garlic, lemongrass, chilli and shallots and rub over the ribs. Add the remaining lime juice, star anise, cinnamon stick and 100ml water to the tin, cover the ribs with a sheet of damp baking paper, then cover the tin with foil. Cook for 3 hours or until very tender, teasing open the foil and paper, then turning the ribs halfway through. Alternatively, cook in a slow cooker on high for 4 hours or low for 6 hours.
While the ribs cook, make the salad. Halve the cucumbers lengthways and scrape out the seeds with a teaspoon. Whack the cucumbers a few times with a rolling pin, hard enough so they split and break up a little, then roughly chop into bite-size pieces. Sprinkle with 1 tsp salt, then set aside in the fridge for 30 minutes. Drain and rinse the cucumbers, then toss with the rest of the salad ingredients. Set aside in the fridge to marinate while you finish the ribs.
Turn up the oven to 220°C fan/gas 9. Remove the ribs from the roasting tin (or slow cooker) and lay flesh-side up in a baking tray lined with foil. Mix the remaining fish sauce caramel with the chilli-garlic sauce and sesame oil, then liberally brush over the rib racks. Return to the oven for 8-10 minutes or until sticky and slightly charred.
Slice the ribs between the bones, sprinkle with coriander leaves and serve with the salad, any remaining glaze on the side and lime wedges for squeezing.