Preheat the oven to 350F and line a baking sheet with parchment paper.
Place the sweet potatoes on a baking sheet and prick all over with a fork. Roast for about 1 hour 30 minutes, or until tender when squeezed (larger sweet potatoes may take longer). Remove from the oven and increase the temperature to 450F.
Meanwhile, make the scallion yogurt. In a medium bowl add the greek yogurt, scallions, garlic, lime juice, olive oil, and salt. Mix and set aside in the fridge.
Smash the potatoes with a spatula or the bottom of a plate until they are about ½" thick. Drizzle generously with olive oil and sprinkle with kosher salt. Return to the oven at 450F for 15-25 mins until the edges are golden brown and crisped.
Serve the sweet potatoes with a generous dollop of yogurt on top of each. Sprinkle with scallions and flaky salt and enjoy!
