How to Make It
In a small saucepan, bring the milk to a boil over moderately high heat. Immediately remove from the heat, stir in the salt and let cool to warm.
Preheat the oven to 325 and butter an 11-by-7-inch glass or ceramic baking dish. In a medium saucepan, bring 1 quart of water to a simmer.
In a large bowl, whisk the cooled milk with the eggs until smooth. Pour the mixture into the prepared baking dish and cover tightly with aluminum foil. Set the dish in a roasting pan and pour enough of the simmering water into the roasting pan to reach about halfway up the side of the baking dish. Transfer the roasting pan to the middle of the oven. Bake the custard for about 30 minutes, until a knife inserted into the center comes out clean. Transfer the baking dish to a rack and remove the foil. Let the custard cool completely, then refrigerate for about 1 hour, until chilled.
Spread the crème fraîche in an even layer over the cooled custard. Spoon the onion, the red caviar and the black caviar over the crème fraîche in alternating diag-onal rows. Sprinkle the caviar cake with the dill, season with pepper and serve.