Leave golf-ball size potatoes whole, slice larger potatoes into halves or quarters. Place potatoes in a heavy-based saucepan.
Add enough cold water to cover, bring to the boil, reduce heat and simmer for 5-6 minutes. Drain and allow to dry.
Place a roasting dish in preheated oven to heat. Remove from oven, add oil and potatoes, lemon slices, garlic and thyme and gently toss to mix.
Roast in preheated oven for 35-40 minutes, turning occasionally, until the lemons are starting to caramelise and the potatoes are golden brown and tender when pierced with a knife.