Preheat oven to 400 °F. Lightly season vegetables with salt and pepper. Fold a piece of parchment paper in half, like a book. Open paper like a book and brush both sides (left and right) with melted butter.
Place the mirepoix down on the paper. Season salmon on both sides with salt and pepper. Place fish on top of vegetables.Drizzle butter and lemon juice on top and fold parchment paper over it to cover.Crimp and fold edges all around to create a seal. When you get to the end, fold it over the bottom.
Bake for 12-15 minutes. Take care when opening parchment because steam will immediately be released. Serve with rice, toasted bread, or mashed potatoes and enjoy.