Preheat the oven to 200°C (180°C fan). Chop any larger potatoes in half.
Put the potatoes in an oven proof roasting dish along with the whole garlic cloves, Oregano and a big pinch of salt and pepper. Drizzle with the olive oil and roast for 15-20 minutes or until starting to soften.
Add the tomatoes, courgette and black olives, toss gently to coat in the olive oil, then return to the oven for a further 15 minutes.
Meanwhile, combine all the dressing ingredients together in a small bowl and whisk to combine.
Remove the tray from the oven and toss through the fresh basil and tarragon. Divide between two plates and serve with the lemon dressing.