Cook the rice according to the package directions.
Heat 1 tablespoon of the oil in a large skillet over medium heat. Add the onion and cook until softened, about 3 minutes.
Add the artichoke hearts, lemon, garlic, and thyme (if desired). Cook until heated through, 3 more minutes. Transfer to a plate.
Season the tuna with the salt and pepper. Heat the remaining oil in the skillet.
Cook the tuna, turning to brown all sides, to desired doneness, about 2 minutes for medium.
Return the artichoke mixture to the skillet and toss. Serve over the rice.