1. In a 3- to 4-quart pan, combine rice, 1/2 teaspoon salt, and 3 cups water. Bring to a boil over high heat, then reduce heat, cover, and simmer until water is absorbed and rice is tender to bite, 20 minutes. Spread rice level in a large baking pan and let cool to room temperature, about 30 minutes.
2. Meanwhile, in a large pan over high heat, bring about 2 quarts water to a boil. Snap ends off green beans and snap or cut beans in half. Add to boiling water and cook just until bright green and tender-crisp to bite, about 2 minutes. Drain, rinse well under cold running water, and drain again.
3. In a small bowl, whisk vinegar, mustard, and garlic until smooth. Add oil and whisk until well blended and thick. Add salt and pepper to taste and 3 tablespoons each of the parsley and chives.
4. Spoon rice into a large bowl (with sealable lid, if planning to transport). Pour dressing over rice and mix to coat. Gently stir in beans, tomatoes, and tuna. Sprinkle with remaining herbs.