Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or silicone baking mat.

In a large bowl, whisk together flour, baking powder, and salt until evenly combined. If making cinnamon bagels, add 2 teaspoons cinnamon at this stage.

Add the Greek yogurt and stir with a spatula until a shaggy, cottage cheese-like dough forms. Switch to using your hands and knead the dough right in the bowl for about 30 seconds until it comes together into a smooth, slightly sticky ball.

Lightly flour your work surface and turn out the dough. Divide into 4 equal portions using a bench scraper or knife. Roll each piece into a ball, then gently flatten and use a spoon handle to create the center hole about 1½ inches wide - it will shrink to about 1 inch during baking.

Beat the egg and brush each bagel generously with egg wash using a pastry brush. Sprinkle with your chosen toppings immediately while the egg wash is wet for best adhesion.

Bake for 22-25 minutes until the tops are golden brown and the bagels sound hollow when gently tapped on the bottom. Let them cool on the baking sheet for 10 minutes before transferring to a wire rack to prevent sogginess.