Preheat the oven to 200°C / 400°F. Halve the carrots lengthwise into long strips (quarter the thicker ones and leave smaller ones whole). Put them on a large baking sheet.
Toss the carrots with the olive oil and some salt. Roast the carrots in the oven for 25-30 minutes, turning once or twice, until tender and caramelized at the ends.
Meanwhile make the crema. Toast the cumin seeds in a small dry skillet over medium heat for about 1 minute. Use the pestle and mortar (or a spice grinder) to finely grind 1 teaspoon of the seeds. In a bowl, whisk the sour cream with the ground cumin, the lime zest and lime juice. If the crema is too thick you can add 1-2 tbsp yoghurt or a little water. Season with salt.
Remove the carrots from the oven and arrange them on a platter. Drizzle over the cumin-lime crema. Garnish with the rest of the toasted cumin seeds and some cilantro.