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 artichokes, lemon, garlic, and thyme (if using). 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 the desired doneness, about 2 minutes for medium. Return the artichoke mixture to skillet and toss to combine. Serve over the rice.
Tip: Look for artichokes packed in water or oil, not marinade. If you can't find them, rinse the artichokes thoroughly with water, then pat them dry.