In a large mixing bowl, combine the flour, baking powder, and salt. Mix well.
Gradually add the water and olive oil to the dry ingredients, stirring until a dough forms. Knead the dough on a floured surface for about 5 minutes until smooth.
Divide the dough into 4 equal pieces and roll each piece into a flat circle about ¼ inch thick.
Heat a skillet or griddle over medium-high heat. Cook each flatbread for about 2-3 minutes on each side until golden brown and cooked through. Keep warm in a clean kitchen towel.
For the tzatziki sauce, in a bowl, combine the Greek yogurt, grated cucumber, minced garlic, lemon juice, salt, and pepper. Mix well and refrigerate for at least 30 minutes to let the flavors meld.
Serve the warm flatbreads with the chilled tzatziki sauce on the side. Garnish with fresh dill or mint if desired.