Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. In a mixing bowl, combine the flour, baking powder, and salt. Add the Greek yogurt to the dry ingredients and mix until a dough forms; it should be slightly sticky but manageable. Lightly flour your work surface and knead the dough gently a few times to bring it together. Divide the dough into 4 equal portions. Roll each portion into a ball, then poke a hole through the center with your finger and stretch it to form a bagel shape. Place the shaped bagels onto the prepared baking sheet. In a small bowl, beat the egg and brush it evenly over the bagels to give them a shiny finish. Sprinkle your choice of optional toppings over the bagels. Bake in the preheated oven for 18-20 minutes, or until golden brown and cooked through. Remove from oven and cool on a wire rack before serving.