Rinse the fish well, pat dry with paper towels and season with salt and pepper.
On medium heat, heat 2 tbsp olive oil in a pan. Add 1 tsp ground coriander seeds and sear the fish skin side down for 4 to 5 minutes, undisturbed (Will vary depending on type of fish and thickness of filets). When the skin is crisp and has a golden color, carefully turn the filets onto the other side using a spatula. Cook undisturbed for another 1 to 2 minutes.
Drain the fried fish on the paper towels before transferring to a serving dish. Keep warm, tented with aluminum foil.
Add a little more oil to the pan if necessary and toast 1/4 cup of coarsely ground hazelnuts. Remove nuts with a slotted spoon and set aside on a plate with kitchen towel to absorb the oil.
In a small bowl, mix the juice of one lemon with 2 tbsp tahini and 1 tbsp water until you get a thick runny sauce and set aside.
Add more oil to the pan and fry the finely chopped onion, the two crushed garlic cloves, the 2 tsp sumac and 1/2 tsp of ground black pepper and cook until the onion is golden brown.
Add the tahini sauce and hazel nuts ( keep a couple of tablessponfuls for garnish) and cook for about 3-4 minutes. Add a little water if the sauce thickens too much. You want to end up with a consistnecy of heavy cream.
Plate the filets, pour sauce over the filets and and garnish with hazelnuts.
Serve with steamed rice, bulgur wheat of freekeh.