In a shallow dish, combine thyme, garlic, EVOO, salt and pepper. Add tuna steaks, flip to coat in marinade then refrigerate at least 1 hour or up to overnight.
Heat a cast-iron skillet over medium-high heat. Cook tuna steaks about 3 minutes on the first side then flip and cook 1-2 minutes more. Remove tuna from pan and slice each steak into pieces about 1/4-inch thick. Douse slices with lemon juice and set aside.