Preheat & prep: Preheat oven to 175 °C (≈ 350 °F). Grease a 9×5-inch loaf pan or line with parchment paper.
Prepare yogurt: If using a loose yogurt, place it in a cheesecloth-lined sieve over a bowl and let drain for 30–60 minutes in the fridge — aim for thick, labneh-like consistency. Measure out 200 g.
Mix dry ingredients: In a medium bowl, whisk together whey protein isolate, almond flour (if using), psyllium husk powder, baking powder, salt — plus xanthan if using — until uniform and no lumps remain.
Combine wet ingredients: In a large bowl, whisk the eggs until blended. Add the drained yogurt, melted butter (or oil), and sweetener if using. Stir until smooth and slightly airy.
Fold dry into wet: Add the dry mixture to the wet in two additions, folding gently with a spatula until fully incorporated. Let the batter rest 3–5 minutes — this allows the psyllium to hydrate and thicken the batter. If batter becomes too thick to spread, stir in 1–2 tablespoons of water or cream to loosen.
Transfer to loaf pan: Spread batter evenly into prepared pan. Smooth the top with a spatula. Gently tap the pan on the counter to release any air bubbles.
Bake: Bake for 35–45 minutes until the top is golden-brown and a toothpick inserted into the center comes out mostly clean. If the top browns too quickly, tent loosely with foil after ~20–25 minutes.
Cool: Let the loaf cool in the pan for about 10 minutes, then transfer to a wire rack. Allow to cool completely (about 30–60 minutes) before slicing — this ensures the crumb sets properly and slicing yields clean slices.