To know Sergio and Lena Martignago is to love them — or so say their longtime customers. That’s why their plan to expand and relocate their restaurant, Sergio’s, has been met with collective enthusiasm. Soon to celebrate their 10th year in business, the Martignagos first opened their doors in November 2002 in the cozy Mill Street space in Grass Valley that had once been home to small food establishments like Heavenly Espresso and Locos Burritos. After a rocky start — which included having to sell their car to meet expenses — Sergio Martignago says a review published in The Union …