Position an oven rack in the center (about 6 inches below the top broiler element) and preheat the oven to 400°F (204°C). Line a rimmed aluminum half-sheet pan with unbleached parchment paper. Pat the 4 salmon fillets (6 oz / 170 g each) thoroughly dry with paper towels, arrange them skin-side down with at least 2 inches (5 cm) of space between fillets, and season evenly with the kosher salt (3/4 tsp / 2.3 g) and black pepper (1/4 tsp / 0.5 g).

In a small glass bowl, whisk together the melted unsalted butter (2 tbsp / 28 g), extra-virgin olive oil (1 tbsp / 15 ml), fresh lemon juice (2 tbsp / 30 ml), finely grated lemon zest (1 tsp / 2 g), minced garlic (4 cloves / 12 g), and Dijon mustard (1 tsp / 5 g) for 20 seconds until the mixture forms a glossy, unified emulsion that will cling to the fish without separating.

Spoon or brush the emulsified lemon-garlic butter evenly over the top and sides of each salmon fillet right before baking (do not marinate ahead, or the lemon acid will chalk the surface proteins). Place 1 thin fresh lemon slice on the corner of each fillet.

Bake uncovered at 400°F (204°C) for 11 to 12 minutes until the sides are opaque coral-pink, then switch the oven to High Broil for 2 to 3 minutes until the garlic-butter glaze bubbles and the edges lightly caramelize. Verify the thickest center reaches 145°F (63°C) on an instant-read thermometer (or pull at 135°F / 57°C for medium-flaky texture and let carry-over heat finish).

Remove the sheet pan from the oven and let the salmon rest for 3 minutes so the warm muscle fibers reabsorb their juices. Spoon the melted lemon-garlic pan juices from the parchment over the fillets, scatter with chopped fresh dill and flat-leaf parsley (2 tbsp / 8 g), and serve warm.