Preheat the oven to 350 degrees F (175 degrees C).

Arrange potatoes in a single layer in a large baking dish or roasting pan.

Mix water, lemon juice, olive oil, mustard, oregano, salt, and pepper together. Pour over the potatoes and stir to coat.

Bake in the preheated oven, carefully turning over halfway through, until potatoes are tender and nearly all excess liquid has evaporated, 75 to 90 minutes.