Enough buttermilk to make the dough into a thick, knead-able consistency
Warm griddle or cast iron skillet in preparation.
Mix the flour, baking soda, and salt in a bowl.
Start with just large splash of buttermilk and when the dough is of a thick, workable consistency it is ready to be cooked.
Dust the griddle with a little dry flour to stop the farl mix from sticking to the pan.
Turn the dough out onto a floured board, knead lightly to form a round shape—then flatten it lightly with a rolling pin. Cut the circle into four or eight wedges and bake them on the griddle, a few wedges at a time.
When the underside has formed a firm skin, turn the wedge over and turn down the heat if necessary as the bread cooks on its second side. After a few minutes cooking on the second side, use a skewer to check if the farl is cooked through. Keep cooking until the skewer comes out clean.
Place egg, lemon juice, and mustard in the bottom of cup or jar that just fits the head of your immersion blender. This is vital. the circumference of the jar must be just larger than the head of your blender and the egg/lemon juice mixture must reach the blades for this to work. If the mixture does not reach the blades, double the recipe before attempting.
Pour oil on top and allow to settle for 15 seconds. Place head of immersion blender at bottom of cup and turn it on high speed. Do not pulse or move the head. As mayonnaise forms, slowly tilt and lift the head of the immersion blender until all oil is emulsified. Season mayonnaise to taste with salt. Store in a sealed container in the refrigerator for up to two weeks.