Preheat the oven to 350°F (175°C). Line the loaf pan with parchment, letting it overhang for easy removal.
In a large bowl, whisk together almond flour, oat fiber, psyllium, baking powder, baking soda, salt, and sweetener (if using). Break up any lumps so the mixture is even.
In a separate bowl, whisk the eggs, almond milk, oil, and vinegar until smooth.
Pour the wet ingredients into the dry.
Stir with a spatula until just combined. The batter will thicken quickly as the psyllium absorbs liquid.
Scrape the batter into the prepared pan. Smooth the top with a damp spatula.
Sprinkle seeds on top if you like.
Bake for 40–50 minutes, or until the top is golden and a toothpick comes out clean. If the top browns too fast, tent loosely with foil.
Cool in the pan for 10 minutes, then lift out using the parchment and transfer to a wire rack. Let it cool completely before slicing for best structure.