Slow-Roasted Salmon with Harissa

1/3 cup extra-virgin olive oil
1/4 cup harissa paste (preferably New York Shuk)
1 garlic clove, grated
1 lemon, halved
1 (2-lb.) skinless center-cut salmon fillet
Kosher salt
Mixed tender herbs (such as parsley, cilantro, dill, and/or chives; for serving)

Preheat oven to 275°F.
Whisk oil, harissa, and garlic in a medium bowl.
Pour half of harissa oil into a 2.5-qt. baking dish and swirl to coat.
Thinly slice a lemon half and remove any seeds; scatter slices in dish.
Season salmon on all sides with salt and place in dish.
Pour remaining harissa oil over salmon, spreading evenly over flesh with a pastry brush or spoon.
Roast 15 minutes. Remove from oven and baste fish with harissa oil pooled in dish.
Return to oven and continue to roast until flesh flakes apart easily with a spoon but is not quite cooked through, 10–20 minutes longer.
Use spoon to break up salmon into irregular pieces.
Arrange salmon and lemon slices on a platter.
Drizzle with any harissa oil left in dish, squeeze remaining lemon half over, and scatter some herbs around.