Preheat the oven to 425°F (220°C). Add 2 pounds potatoes (diced) to a large baking sheet. Toss with 2 tablespoons olive oil, 1 teaspoon salt, and pepper. Spread in a single layer. Roast for 35 to 40 minutes until golden. Let them cool on the tray until warm or at room temperature.
Grate 1½ packed cups cucumber using the large holes of a box grater. Squeeze out most of the water with your hands.
In a large bowl, combine the squeezed cucumber, 1 cup Greek yogurt, 2 tablespoons tahini, 1 tablespoon olive oil, 2 tablespoons lemon juice, 1 to 2 cloves garlic (grated), 2 green onions, ½ cup dill (chopped), some salt, and pepper. Stir until smooth and creamy.
Add the cooled potatoes to the bowl. Toss gently to coat. Taste and adjust with more salt or lemon juice if needed. Serve right away.
