The owners of Tani Sushi Bistro in Clayton have opened one of the cornerstone restaurants of the new Mercantile Exchange development downtown. Like Tani, Takaya New Asian offers nigiri sushi, sashimi and over-the-top Americanized rolls. (The “Oh My God” roll comes to your table engulfed in flames.) Unlike Tani, though, Takaya fails as a restaurant in nearly every respect. Much of the menu features dubious takes on the already played-out Asian-fusion trend. There are sliders with bulgolgi beef, and fried cheese sticks classed up with a light tempura breading. The sushi is sloppily prepared, only one of several missteps from a careless kitchen that might mar your meal.