Preheat the oven to 400F. Scale the trout fillets and check that there are no bones in them. Keep the fish refrigerated until ready to stuff.
Prepare all the ingredients for the stuffing paste. Peel and finely dice the onion. Sprinkle it with 2 tbs kosher salt. Let sit for 10 minutes, rinse under the running cold water, and squeeze extra water and juices. Peel and finely dice garlic cloves. Wash, slit open, remove the seeds and membranes and dice the serrano. Wash and chop cilantro. Measure the rest of the ingredients.
Place all stuffing paste ingredients into a blender jar and process them into a paste. There is no need to make it smooth; it should be slightly grainy, yet homogenous. Taste it and adjust its seasoning to your taste.
Pat dry trout fillets with a paper towel. Arrange half of them on a baking pan lined with a silicone mat or parchment paper with their skin down. Spread the paste 0.5 inches thick and cover with the rest of the fillets, their skin up. Make a few shallow slits on the trout skin and truss with a culinary tread or blanched scallions/chives. Spray the trout with walnut oil. You can prepare the fish in advance, cover, and keep refrigerated for at least 30 minutes or up to 2 hours until ready to roast.
Roast fish for 20-25 minutes until it's fully cooked and broil for 1 minute at the very end.
Top with fresh lemon slices and pomegranate seeds (if in season) and serve with rice, roasted or boiled potatoes, or blanched greens.