How to Make It
In a large pot of boiling, salted water, cook the rice until just done, about 10 minutes. Drain. Rinse with cold water. Drain thoroughly.
In a medium glass or stainless-steel bowl, combine the rice, onion, tomato, bell pepper, olives, parsley, and 1/2 teaspoon of the thyme. In a small glass or stainless-steel bowl, whisk the vinegar with the mustard, garlic, 1 teaspoon of the salt, and 1/2 teaspoon of the black pepper. Add the 1/3 cup oil slowly, whisking. Toss this vinaigrette with the rice.
Rub the tuna with the remaining 1 tablespoon oil and sprinkle with the remaining 1/2 teaspoon thyme, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Heat a grill pan or large heavy frying pan over moderately high heat. Put the tuna in the pan and cook for 2 minutes. Turn and cook until done to your taste, about 2 minutes longer for medium rare. Mound the rice on plates and top with the tuna.