Preheat the oven to 350°F (175°C). Line a baking sheet with aluminum foil.
Place the salmon skin-side down on the lined baking sheet and drizzle with olive oil, then season with salt and pepper.
Fold up the foil edges around the salmon to create a shallow packet, leaving the top open for now.
In a medium bowl, whisk together brown sugar, soy sauce, honey, minced garlic, stone-ground mustard, orange zest, lemon juice, and orange juice until smooth. Do not add the butter yet.
Pour most of the glaze over the salmon, reserving a little for basting. Dot the top of the fish with the optional butter pieces.
Seal the top of the foil packet and bake in the preheated oven for 20–25 minutes, until the salmon is cooked through but still moist. Cooking time will vary by thickness—check for flakiness.
Remove the foil top, baste the salmon with the reserved glaze and any melted butter, then switch the oven to broil. Broil 2–3 minutes until the glaze is caramelized and browned—watch carefully to avoid burning.
Let the salmon rest a minute, then serve with salad or vegetables as desired.
