Place the potato wedges in a bowl of ice water and soak for 30 minutes.
While the potatoes soak, preheat the oven to 425°F (218°C) and move the oven rack to the middle position.
In a small bowl, combine 1 teaspoon garlic powder, 1 teaspoon dried oregano, 1/2 teaspoon paprika, 1/2 teaspoon onion powder, salt and pepper to taste, the juice of 1/2 lemon, and 2 tablespoons olive oil. Stir until blended.
Drain the potato wedges and pat them dry thoroughly with paper towels or a clean kitchen towel.
Put the dried wedges into a large zip-top bag or bowl, pour the seasoning mixture over them, seal (if using a bag) and toss until the wedges are evenly coated.
Arrange the seasoned wedges in a single, even layer on a baking sheet (do not overcrowd).
Bake for 25–30 minutes, flipping the wedges once about halfway through the baking time (around 12–15 minutes), until they are golden brown and tender when pierced.
While the wedges bake, prepare the tzatziki.
Remove the wedges from the oven and transfer to a serving platter. Top with tzatziki (or serve it on the side for dipping), sprinkle with crumbled feta, add chopped red onion and chopped fresh parsley to taste, and serve with lemon wedges for extra squeezing.