Step 2

Dice the remaining avocado and fold it through with most of the lime zest. Keep the pieces distinct, then cover and chill until you need it.

Step 3

Boil the potatoes until a knife meets only slight resistance, around six minutes. Drain them and let the steam die off in the colander.

Step 4

Toss the warm potatoes with the olive oil, chili powder and cumin seeds. They should be dry enough that the spices cling instead of sliding off.

Step 5

Roast at 400°F (fan 350°F) for about thirty minutes, shaking the tray once. They are ready when the skins blister and a fork slides in unopposed.

Step 6

Spoon the dip into bowls and scatter the rest of the lime zest over. Bring it to the table while the potatoes are still hot, with the tortilla chips.