Preheat the oven. Set it to 400°F (200°C).
Line a large baking sheet with parchment paper or lightly oil it.
Pat the salmon dry. Use paper towels to remove surface moisture. This helps the fish roast, not steam, and keeps the texture tender.
Make the lemon herb mixture. In a small bowl, combine softened butter, olive oil, lemon zest, lemon juice, garlic, parsley, dill, chives, thyme, salt, pepper, and red pepper flakes if using. Stir until it forms a loose paste.
Season the salmon. Place the fillet skin-side down on the baking sheet.
Spoon the lemon herb mixture over the top, spreading it evenly. Lay a few thin lemon slices on top for aroma and presentation.
Add optional sides. If roasting vegetables on the same pan, toss them with a bit of olive oil, salt, and pepper. Arrange around the salmon in a single layer.
Roast to just done. Bake for 10–14 minutes, depending on thickness.
A general guide is 4–6 minutes per 1/2 inch of thickness. The salmon should flake easily with a fork and be slightly translucent in the center.
Rest briefly. Remove from the oven and let it sit for 3 minutes. This helps the juices settle and keeps the fish moist.
Finish and serve. Sprinkle with a little more chopped parsley and a squeeze of lemon if you like.
Serve straight from the sheet pan or transfer to a platter.