Keep two steaks of tuna aside at room temperature.
Combine 2 cloves garlic, 6 sprigs thyme, and a pinch of red pepper flakes in a heavy skillet. Pour 2 cups olive oil into a skillet to reach a depth of 1 inch. Heat oil over medium heat until garlic and thyme begin to sizzle, 5 to 10 minutes.
Place tuna gently in hot oil and reduce heat to low. Cook steaks in hot oil, spooning oil over the top of the steaks constantly, until fish is white and hot, 5 to 7 minutes. Remove from heat, transfer steaks to a baking dish, and pour hot oil and herbs over the fish. Cool fish and oil to Room temperature
Wrap baking dish tightly in plastic wrap and refrigerate steaks for 24 hours. Remove fish from oil and sprinkle sea salt over the top.