Put the yams in a medium saucepan and cover with water. Put the saucepan over high heat and bring to a boil. Lower the heat to medium and cook until the yams are tender.
Preheat the oven to 400°F/200°. Line a baking tray with baking paper.
Peel and slice the onion. Chop the preserved lemons finely or zest the lemon or lime.
Place the yams on the baking tray, put a spatula over each yam and smash it. Sprinkle over the onions, preserved lemons or zest the lemon and salt. Drizzle a little bit sesame oil and tamari, and put the tray in the middle rack of the oven.
Roast until the onions are crispy and the yams' skin is golden brown, about 10-15 minutes. Place the smashed yams on a plate, squeeze lime juice over, garnish with scallion and serve immediately.