2) Whilst the burgers are resting, in a large saute pan heat 55g of the butter and 1 tbsp of the olive oil. Add all the onions and saute for 15 to 20 minutes or until nicely caramelised. Set aside, but keep warm.
3) In the same pan in which the onions were sauteed, add 3 more tbsp of the olive oil and the mushrooms and saute for 6-8 minutes over a medium high heat until softened. Set aside, but keep warm.
4) Heat a griddle or cast iron skillet to a medium high heat, add the rapeseed oil and spread evenly. Put the turkey burgers on the griddle and cook for 4 to 5 minutes per side. Remove from the heat, cover and keep warm.
5) Butter the rye bread lightly and add to the pan, butter-side down. Place a slice of Gruyere on the bread, then place a turkey burger on the cheese, pile with onions and add the other piece of rye. Repeat for all four burgers.
6) After 2 to 3 minutes, turn the sandwich over carefully and brown on the other side. Remove to a cutting board and cut on the diagonal. Serve with mustard and hot cherry peppers.