Heat oven to 350 F.
Season both sides of the salmon fillets with salt and pepper, then liberally coat the non-skin side of the salmon with Cajun seasoning (use as much as necessary to cover the entire surface of the fish).
To make the pico de gallo: Mix together the green onions, heirloom tomato, cilantro, jalapeño and lime juice. Season with a pinch of salt.
Preferably under a stove exhaust hood or fan, heat a heavy bottom or cast-iron pan until very hot. Add 1 tablespoon olive oil. Carefully add the salmon fillets, Cajun seasoning side down. When the spices have blackened (after only 2 to 3 minutes), turn the fish over and place the pan in the oven. Using an instant-read thermometer to check the internal temperature, remove the fish from the oven when it reaches 115 F for medium rare, and let it rest for 3 to 5 minutes before serving.
Divide the spring mix between serving plates. Place salmon fillets on top and spoon pico de gallo liberally over the fish. Top with avocado and drizzle with olive oil. Add sea salt and freshly ground black pepper to taste.