Mix together the oatmeal, wholemeal flour, plain flour, yeast and salt in a bowl until well combined.
Make a well in the centre of the mixture, then gradually add the water in a thin stream, stirring well with a wooden spoon, until the batter is well combined and the consistency of thick double cream. (NB: You may not need to use all of the water.)
Cover the batter with a damp, clean tea towel and set aside for 3-4 hours (or up to 8 hours), to allow the gluten in the batter to develop.
Once ready to cook the batter, whisk in the baking powder until well combined.
Heat one teaspoon of the vegetable oil in a frying pan over a medium heat. When the oil is hot, add a ladleful of the batter mixture and swirl the pan to coat the bottom of it in a thin layer of the batter.
Fry the oatcake for 1-2 minutes, or until golden-brown on one side.
Flip the oatcake and fry for a further minute, or until golden-brown on both sides.
Set the oatcake aside on a warm plate and cover with a sheet of greaseproof paper.
Repeat the process with the remaining batter mixture, to make about 12 oatcakes. Stack them on top of each other, separated by greaseproof paper, and keep warm.
Meanwhile, preheat the grill to its highest setting.
When the grill is hot, arrange the streaky bacon onto a grill tray and grill for 3-4 minutes on each side, or until crisp and golden-brown on both sides. Set aside and keep warm.
Meanwhile, heat the olive oil in a separate frying pan over a medium heat.
Add the mushrooms and fry for 3-4 minutes, or until golden-brown and cooked through. Set aside and keep warm.
Meanwhile, heat a frying pan over a medium heat until hot. Return one of the cooked oatcakes to the hot pan and sprinkle grated cheese over one half of it.
Continue to cook for 3-4 minutes, or until the cheese starts to melt, then place two grilled rashers of bacon on top of the melted cheese and sprinkle over a spoonful of the mushrooms. Fold the other half of the oatcake over and continue to heat for a further minute, or until the cheese has fully melted.
Repeat the process with the remaining vegetable oil, oatcakes, bacon, mushrooms and cheese.