Preheat oven to 450 degrees. Combine 4 tablespoons olive oil, lemon juice, 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/4 teaspoon oregano, 1/4 teaspoon thyme, 1/4 teaspoon basil, and 1/2 teaspoon garlic powder in a large resealable bag along with the tuna fillets. Seal the bag, massage the fillets to rub in the seasonings and distribute all the ingredients. Chill for 15 minutes while you move on to the next step.
In a large bowl combine potatoes and remaining 2 tablespoons olive oil and toss to coat. Stir together remaining salt, pepper, oregano, thyme, basil, and garlic powder, sprinkle over potatoes and stir to combine. Arrange potatoes on half of a greased sheet pan. Bake for 15 minutes.
After 15 minutes, place tuna fillets on the empty half of the sheet pan (add lemon slices if desired), sprinkle parmesan cheese over the potatoes and return to the oven for 6 minutes. Flip the tuna fillets, and cook for another 6 minutes. Serve and enjoy!