Wash and pat dry sea bass fillets.
Drizzle with fish sauce, lemon juice and add salt and pepper, to taste, on both sides.
Heat cast iron skillet, over high heat, until almost smoking.
Reduce heat to medium high and add 1 teaspoon avocado oil.
When hot, add sea bass fillets, in a single layer.
Cook for 4 - 6 minutes, per side, without touching them until browned and cooked through.
Remove cooked fish and set aside.
In the same pan, cook the spinach in 1 teaspoon sesame oil until just wilted; about 1 minute.
To plate, first divide spinach onto four plates, sprinkle with sesame seeds, top with sea bass, and drizzle with sauce.