Begin by briefly sauteing the portabella mushroom slices and 1 tsp of butter in a cast iron skillet. Cook until softened, but not fully cooked. Set aside to cool.
Meanwhile,prepare compound butter by combining softened butter (remaining 4 tbsp), Thai red curry paste, Thai curry seasoning and grated garlic. Mix well until fully combined. Place butter mixture on a square of plastic wrap, roll tightly into a log and refrigerate 10-15 minutes.
When ready to cook, set grill temperature to 400° and preheat, lid closed for 15 minutes. Take one square of Traeger paper or parchment and place on the center of your cutting board.
Place½ of the sauteed mushrooms on the center of the paper. Top with a portion of halibut and season lightly with salt. Top halibut with Serrano slices,scallions, and herbs.
Cut prepared compound butter into quarters. Place two pieces of compound butter on top of halibut.
Bring sides of Traeger paper together, and fold to seal. Bring in sides of Traeger paper to create a tightly-sealed packet. Place prepared packets on a small grill-proof cookie sheet. Place cookie sheet in grill and cook for 15-minutes until cooked through.
Remove from grill and place each packet on a dinner plate with a serving of sticky white rice. Each diner can tear open their packet, revealing perfectly cooked halibut in a spicy Thai sauce.