In a large bowl, combine flour, salt, and baking soda. Gently stir in the buttermilk just until dough comes together.
On a lightly floured surface, pat the dough into a circle 1/2-3/4 inch (1.25-2 cm) thick. Use a sharp knife or pizza cutter to cut the circle into 4 equal quadrants.
Place a large skillet over medium heat. Lightly grease with oil or butter. Once heated, add the cut farls to the skillet, making sure they don't touch. If the pan is too small, cook them in 2 batches. Pan-fry until the bottom is browned, about 6 minutes, then flip and cook until browned and heated through.
Serve warm with butter and jam.