Cook onions in 1 tbsp oil until soft and translucent for around 10 minutes. Add an extra 5 minutes if you like them to be a little golden.
In a bowl, add the breadcrumbs, 2 tbsp oil, 2 flax eggs, sage and 1 tsp salt along with the cooked onions.
Using clingfilm, shape into a log, wrap tightly then chill in the fridge for 1 hour.
In the mean time, preheat oven to 200°C.
In a bowl, combine ½ tsp salt with maple syrup, 1 tbsp and smoked paprika.
Add the mushroom slices and mix thoroughly to coat evenly with the maple mix.
Lay flat on a baking tray, ensuring they are not on top of each other and bake in the oven for 15-20 minutes, flipping halfway.
Remove stuffing from fridge and unwrap.
Lay one layer of slices on top of the log and tightly wrap the entire stuffing log with foil, ensuring edges are also sealed.
Bake at 200C for 30 minutes.
Remove the foil and bake another 5 minutes before removing from oven.