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1
Preheat the oven to 300 degrees.
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2
Place tomatoes on a baking sheet and sprinkle with half the garlic; season with thyme, sugar, about 1/4 teaspoon salt and 1/8 teaspoon pepper.
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3
Bake until the tomtoes are shriveled, about 3 to 3 1/2 hours; let cool.
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4
Coarsely chop the tomatoes.
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5
Then heat the teaspoon of olive oil in a skillet over medium heat; add remaining garlic and cook until softenend, about 2 minutes.
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6
Stir in the tomatoes and tarragon, cook for 2 minutes more; taste and add salt and pepper as desired.
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7
Meanwhile, in a medium saucepan, over medium high heat, bring the water, vinegar, sugar, 1/2 teaspoon salt and 1/4 teaspoon pepper to a boil; add onion and cook until it just begins to soften, but is still crisp, about 5 minutes.
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8
Drain and let cool, then toss with olive oil and set aside.
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9
Heat about a quart of olive oil (for poaching the fish) to 140 degrees F (use an instant read thermometer), in a medium saucepan.
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10
Pat the fish dry with paper towels and season with salt and pepper.
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11
Slide fish into the oil and cook until it is pale grayish on the outside, but still pink inside, about 5 minutes.
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12
To serve, divide the pickled onions among 4 plates; top with tomato sauce, then tuna seasoned with additional salt and pepper, if desired.
