Mix garlic, sesame oil, sugar, soy sauce and vinegar in a small bowl. Set aside for 10-15 minutes while you prep everything else to allow the garlic to mellow in the vinegar.
Start preparing the cucumbers by trimming the ends. Place each cucumber on a wooden chopping board. Smash it gently along the length using a heavy pestle or rolling pin until it splits. Slice the cucumbers in 1-2 inch diagonal slices. Using a spoon, gently remove some of the wet seed clumps. Scoop the cucumber pieces into a medium bowl. Sprinkle with a pinch of sea salt flakes and black pepper. Set aside for 15 minutes. Then drain off the excess liquid.
Prepare the chilli oil by heating the grapeseed oil in a small saucepan on low heat. Add chilli flakes and Gochugaru. Continue cooking on a low heat for 5-6 minutes until the oil is toasted and fragrant. Remove from heat.
Pour dressing over the cucumber and mix gently until cucumber is coated.
Pour the chilli oil over the cucumber. Sprinkle sesame seeds and garnish with spring onion. Serve immediately.
