Heat grill to medium hot. Brush the onions with 1 teaspoon olive oil, and sprinkle to taste with salt and pepper. Cook on the grill until soft and golden, 3 to 5 minutes per side. Remove from grill, and let cool. If you dont have a grill, cook onions under a hot broiler. Meanwhile, fill a medium saucepan with cold water, and add salt and potatoes. Set pot over high heat, and bring to a boil. Reduce heat; simmer until potatoes are fork tender, 10 to 14 minutes. Drain potatoes, let stand until cool enough to handle, and rub off as much skin as possible. Trace a circle with the point of a knife extending about 1 inch beyond edges of tomato core. Using a sharp knife held on a diagonal, cut along circle, removing core in one cone-shaped piece; discard. Slice the potatoes into long halves or quarters, depending on size and type used. Combine the onions, potatoes, capers, arugula, olive paste, olives, 1 tablespoon olive oil, lemon juice, and balsamic vinegar in a large mixing bowl; toss. Crumble in the tuna, and toss gently. Adjust the seasoning to taste with salt and pepper. Season the cavity of each tomato with salt and pepper. Fill each tomato with tuna mixture. These will hold well at room temperature for at least 1 hour. Makes 6 servings. Source: "Martha Stewart Living Magazine, Jul/Aug 1997" S(Formatted for MC5): "by Lynn Thomas - Lynn_Thomas@prodigy.net" Per serving: 35 Calories (kcal); 3g Total Fat; (76% calories from fat); trace Protein; 2g Carbohydrate; 0mg Cholesterol; 766mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 1/2 Vegetable; 0 Fruit; 1/2 Fat; 0 Other Carbohydrates Recipe by: Martha Stewart Converted by MM_Buster v2.0n.