Transfer half a cup of the water to a small bowl and heat up until the temperature is between 105°F and 115°F. Add the sugar and yeast, stirring just until combined. Stir briefly and then set aside for the yeast to activate. When the yeast is bubbly and risen, after about 5 minutes, proceed with the recipe.
Use a food processor or blender to grind the oat into a coarse flour. In a large mixing bowl, combine the finely ground oats, all-purpose flour, and salt. Whisk together. Add the yeast mixture along with the remaining half cup water and the milk. Whisk until it makes a smooth, thin batter. Cover with a tea towel and let rise for 1 hour.
Melt the butter in a small dish and set aside with a pastry brush. Gently stir the batter and check the consistency. The batter should be thin enough to pour easily from a ladle, similar to crepe batter. Gently whisk in an additional 3 or 4 tbsp of water, as needed to make a thin batter.
Place a non-stick skillet over medium heat. When the skillet is hot, brush quickly with some of the melted butter. Add one ladleful of the batter to the skillet. Tilt the pan from side to side to make a thin pancake, similar in size and shape to a crepe. If the batter does not spread out into a thin crepe, then add a bit more water.
Allow the oatcake to cook until it is almost fully cooked. If you try to flip them too early, the oatcake might fall apart. When the top of the oatcake looks almost fully set, flip the cake over. Let cook for another 1 to 2 minutes until the oatcake is fully cooked and lightly browned on both sides. Transfer to a plate and cover to keep warm. Repeat with the remaining batter.
Serve these oatcakes with a savoury dish such as stew, or slathered in butter and honey. Enjoy!