Arrange racks in the middle and at the top of the oven, then preheat it oven to 320°F (160°C). Line a baking sheet with parchment paper.
roast the salmon
For the salmon, place the fillet on the prepared baking sheet. Season it with salt and pepper, then drizzle it with the oil. Slow-roast the salmon on the middle rack of the oven for 14 to 16 minutes, or until a thermometer inserted into the thickest part of the fish registers 118°F (48°C).
make the glaze and topping
While the fish is roasting, make the honey mustard glaze. In a bowl, stir to combine the Dijon mustard, American mustard, honey, and lemon juice. For the topping, in a bowl, combine the chives, dill, oil, and lemon zest and juice.
glaze the salmon and broil
Remove the fish from the oven. Change the oven temperature to high broil. Brush the salmon generously with the honey mustard glaze. Roast the salmon on the top rack of the oven, under the broiler, for 2 to 4 minutes, or until golden on top.
To serve, spoon the herb topping over the roasted salmon fillet, then sprinkle it with the chile, if using. Serve the dish with your favorite side.