Prep the pan and oven: Preheat to 350°F (175°C). Line a 8.5 x 4.5-inch loaf pan with parchment and lightly grease.
Mix dry ingredients: In a large bowl, whisk flaxseed meal, baking powder, baking soda, salt, and cinnamon.
Break up any clumps.
Whisk wet ingredients: In a separate bowl, whisk eggs, olive oil, almond milk, vinegar, and sweetener (if using) until smooth.
Combine: Pour wet into dry. Stir until thick and uniform. If using chia or hemp hearts, fold them in.
The batter will be dense but spreadable.
Rest briefly: Let the batter sit 3–5 minutes so flax absorbs liquid. It will thicken further.
Fill the pan: Scrape batter into the pan and smooth the top. For a nicer rise, mound the center slightly.
Bake: Bake 40–50 minutes, until the top is firm and a toothpick comes out clean.
If the top browns too quickly, tent with foil for the last 10 minutes.
Cool completely: Rest in the pan 10 minutes, then transfer to a rack. Cool at least 1 hour before slicing for best structure.
Slice and serve: Use a sharp serrated knife. Toasting enhances flavor and firms the crumb.