Prep the oven and pans: Heat the oven to 325°F (160°C). Set a kettle of water to boil. Arrange ramekins or the baking dish inside a deep roasting pan.

Make the caramel: In a small saucepan, add sugar, water, and lemon juice. Cook over medium heat without stirring until the syrup turns a deep amber, 6–9 minutes. Swirl the pan gently if needed, but don’t stir. Immediately pour the caramel into the ramekins or baking dish, tilting to coat the bottoms. Set aside to firm up.

Blend the custard base: In a bowl, whisk eggs until smooth. Add milk, Greek yogurt, vanilla, sugar, and salt. Whisk until combined and no yogurt lumps remain.

Add the protein powder carefully: Sprinkle the protein powder over the custard while whisking. Mix just until smooth. Avoid over-whisking to prevent foam and bubbles.

Strain for silkiness: Pour the custard through a fine-mesh sieve into a large measuring cup. This step removes bubbles and bits that cause a grainy texture.

Fill the molds: Divide the custard evenly over the set caramel. Skim any surface bubbles with a spoon.

Set up the water bath: Slide the roasting pan into the oven. Carefully pour hot water into the pan until it reaches halfway up the sides of the ramekins or baking dish.

Bake gently: Bake 30–40 minutes for ramekins, 45–60 minutes for a larger dish. The flan should be mostly set with a slight jiggle in the center. Do not overbake.

Cool gradually: Remove from the oven and let the flan sit in the hot water for 10 minutes. Transfer ramekins to a rack to cool to room temperature.

Chill to set: Cover and refrigerate at least 4 hours, ideally overnight. The caramel will liquefy and create a sauce.

Unmold and serve: Run a thin knife around the edge, place a plate over the ramekin, and invert. Let the caramel sauce flow over the flan. Spoon extra sauce on top.