Let tuna rest at room temperature for 10 to 15 minutes.

Combine garlic, thyme, and red pepper flakes in a heavy skillet. Pour olive oil into the 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 steaks gently in hot oil and reduce heat to low; cook, spooning oil over the top of tuna steaks constantly until white and hot, 5 to 7 minutes. Remove from heat and transfer tuna steaks into a baking dish; pour hot oil and herbs on top. Cool tuna and oil to room temperature.

Wrap the baking dish tightly in plastic wrap and refrigerate tuna steaks for 24 hours; remove from oil and sprinkle sea salt on top.