In a skillet heat 2 tablespoons oil over moderately high heat until hot. Add onions, salt, 1/2 teaspoon pepper, and thyme and saute until rich golden in color. Heat another skillet over moderately high heat until hot and add remaining 2 tablespoons oil. Swirl and add snapper. Saute for 2 minutes, turning frequently, add garlic, lime juice, and soy sauce and saute until liquid is nearly evaporated and snapper is lightly golden in color. Add oregano and remaining pepper and toss to combine. Add onion mixture and toss well. Fill tortillas with lettuce and top with snapper and onion mixture.