TMZ got an earful from the woman who served Nick and Mariah at LA's Gyu Kaku -- a restaurant where you cook your own meat!
According to the server, Nick, Mariah and one of her backup dancers came in for an intimate dinner earlier this week. Mariah's bodyguard came too, but he was banished to his own table.
We're told Mariah and the dancer sipped on wine while Nick, being the eternal 12-year-old, guzzled four Shirley Temples. The couple held hands throughout the meal and seemed "down to earth. "
And for the piece de resistance - Nick paid! And left a 20% tip.