How to Make It
Preheat the oven to 325°. Butter four 1/2-cup ramekins and set them in a baking dish that holds them snugly.
In a large saucepan, melt the butter. Add the onion and garlic and cook over moderate heat, stirring occasionally, until softened, about 10 minutes. Add the water and bring to a boil. Cover and simmer over moderately low heat until the onion is very tender, about 25 minutes. Uncover and boil over high heat until any excess liquid has evaporated, about 2 minutes.
Set a food mill fitted with the fine disk over a medium bowl. Transfer the onion mixture to the mill and puree. Whisk the eggs, cream, salt and pepper into the puree. Pour the onion custard into the prepared ramekins. Add enough tepid water to the baking dish to reach halfway up the sides of the ramekins. Bake the flans for 35 minutes, or until almost firm but still slightly loose in the center; if the water in the baking dish simmers, add a few ice cubes to cool it down.
In a medium saucepan, heat the olive oil. Add the onion and garlic and cook over moderate heat, stirring, until fragrant, about 2 minutes. Add the tomatoes, thyme and oregano sprigs and salt and pepper, cover and cook over moderate heat for 10 minutes. Set a food mill fitted with the coarse disk over a medium bowl. Add the tomato sauce and work it through the mill. Transfer the tomato sauce to a small saucepan and keep warm.
Remove the ramekins from the water bath and wipe the bottoms and sides dry. Run a small, sharp knife around each flan to loosen it, then invert the flans onto plates. Spoon the tomato sauce around the flans and sprinkle with the chives. Serve right away.