Wash and pat dry sea bass fillets. Drizzle with fish sauce, lemon juice and add salt and pepper to taste on both sides (optional)
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.