Halve the sweet potatoes lengthwise. Rub the cut sides with oil, season with salt. Place cut-side down on a sheet pan. Roast at 425°F for 25-30 minutes until the cut side is deeply caramelized and a knife slides through without resistance.
Dice the tomato, slice the red onion thin. Slice the cucumber into coins or half-moons. Warm the frozen pitas directly on a rack in the oven for the last 5 minutes of the sweet potato roast, or in a dry skillet.
Whisk Greek yogurt with a squeeze of lemon juice, a pinch of salt, and a thin drizzle of olive oil until loose and spoonable.