Heat the oven to 200 C / 400 F. Halve the leeks lengthwise, slice them into 1 cm half-moons, and swish them in a bowl of cold water to loosen the grit. Lift them out with your hands, pat dry, and peel and chop the potatoes into 2.5 cm chunks.

Tip the leeks, potatoes, whole unpeeled garlic cloves and thyme sprigs onto a 33 x 46 cm / 13 x 18 inch sheet pan. Add the olive oil, 1/2 tsp of the salt and the pepper, toss well, and spread everything in a single layer.

Roast for 35 minutes, stirring once at the 20-minute mark, until a fork slides easily into the potatoes and the leeks have golden-brown edges. Dark brown tips are fine. Pull out and discard any pieces that have gone black, since they taste bitter.

Squeeze the soft garlic out of its skins into a 4-quart / 3.8 L soup pot and discard the skins and thyme stems. Add the roasted vegetables and the stock, using a splash of it to scrape the browned bits off the pan. Bring to a simmer over medium heat and cook for 10 minutes.

Blend until smooth, 1 to 2 minutes with an immersion blender. With a countertop blender, work in batches and fill it no more than halfway, with the lid vented. Stir in the lemon juice and the remaining 1/2 tsp salt, thin with a splash of water if it's too thick, and taste. Serve hot, topped with chives.