Preheat the oven to 375°F (190°C). Line a baking sheet with a large piece of aluminum foil.
Place the salmon on the foil skin-side down with the long side parallel to the sheet; fold up the foil edges about 2 inches to form a shallow well to hold the glaze.
Tuck the lemon slices underneath and around the salmon so they are evenly spaced, then set the fish aside.
In a microwave-safe bowl or small saucepan, melt the butter. Stir in the honey, Dijon mustard, and lemon juice until smooth.
Pour or spoon about two-thirds of the butter-honey mixture over the salmon, reserving the rest for later. Season the salmon evenly with kosher salt and freshly ground black pepper.
Seal the foil packet by folding or crimping the top and bottom pieces together so it is as airtight as possible. If desired, let it sit to marinate 10–15 minutes for extra flavor.
Bake the sealed salmon in the preheated oven for about 15 minutes.
Remove the baking sheet from the oven and open the foil packet so the salmon is exposed but the foil edges still contain the juices.
Set the oven to high broil. Spoon the reserved buttery mixture over the top of the salmon if needed, then broil for 5–10 minutes until the top is golden and slightly caramelized—watch closely to avoid burning.
Remove from the oven, garnish with chopped parsley if using, and serve immediately.
