Heat the butter and stir in the diced mushrooms and onions.
Add rosemary and seasoning and stir to combine.
Lay green spinach on flattened pork tenderloin and top with mushroom mixture.
Roll the tenderloin over the filling and secure with twine.
Grill for 40 minutes, turning occasionally.
Brush with barbecue sauce and grill another minute, then let rest and slice.