Russell Westbrook had the best Tuesday EVER -- because after he scored a $47 MILLION payday from the Lakers ... he celebrated with Kevin Hart at an L.A. hotspot!!

Russ opted into the final year of his Lakers contract early Tuesday morning -- making him richer than ever -- and hours later, he, Hart and some other buddies grabbed dinner at LAVO Ristorante, a swanky West Hollywood joint.

The two were in great spirits -- laughing and dappin' each other up outside of the eatery ... and while Russ didn't say too much, it's pretty clear he's fired up to be staying in L.A. for at least another year.