Heat the veal stock and reserve. Heat 1/3 of the olive oil in a saucepan over medium heat, add the rice and stir for 1 minute. Add 1/3 of the veal stock with the dried porcini, thyme, pepper and salt, and cook over medium heat until the liquid is fully absorbed, stirring constantly. Add another 1/3 of the liquid and simmer over low heat until fully absorbed, still stirring regularly. Remove from heat, cover with a lid and let rest for 10 minutes.
Transfer the risotto to a blender, add the heavy cream, the rest of the olive oil and the remaining veal stock, and process until smooth. Pass through a chinois, transfer to a plastic container and refrigerate.
Place the veal tenderloin in the freezer for about 30 minutes.
In a saucepan over medium heat, sauté the onions with half of the butter, season with salt and cook until golden brown but still slightly crunchy. Reserve.
In another saucepan over medium heat, sauté the mushrooms in the rest of the butter, season with salt and pepper, and cook until the rendered liquid is fully reduced. Reserve.
Finely grate the parmesan and reserve.
Chop the veal tenderloin into a fine brunoise. Arrange on a bed of risotto cream, topped with an egg yolk and surrounded with the garnishes. The dish should be cool but not too cold. Have bottles of Worcestershire sauce, Tabasco sauce, olive oil and balsamic vinegar on hand to let your guests finish the tartare to their liking.