For the roasted garlic-oregano vinaigrette:
1) Combine garlic, vinegar, oregano, parsley, honey and salt in a blender and blend until smooth. With the motor running, slowly add the oil and process until emulsified. Stir in the red chilli flakes.
For the grilled chicken and potatoes:
1) Place potatoes in a medium saucepan, cover with cold water and add 1 tbsp of salt. Bring to a boil over high heat and cook until a paring knife inserted comes out with some resistance. Do not cook the potatoes all the way through because they will continue cooking on the griddle. Drain well and when cool enough to handle, slice in half lengthwise.
2) Heat the griddle to medium.
3) Brush the chicken and potatoes with oil and season with salt and pepper. Place the chicken on the griddle, skin-side down and griddle until golden brown and slightly charred, 6 to 7 minutes. Turn the chicken over and continue to griddle until just cooked through, 5 to 6 minutes. A few minutes before the chicken has finished cooking, place the potatoes on the griddle, cut-side down and cook until lightly golden brown, about 2 minutes. Turn over and continue to griddle, about a minute longer. Remove the chicken and potatoes to a platter and immediately drizzle with the roasted garlic-oregano vinaigrette. Let rest 5 minutes before serving. Garnish with oregano sprigs and parsley sprigs.