Andy Ruiz Jr. ain't one to let a little thing like a boxing title change up his dining preferences -- the dude digs authentic Mexican ... even if it means getting it from a hole-in-the-wall joint.
The newly-minted heavyweight champ -- who recently stunned the world by defeating the heavily-favored Anthony Joshua -- was spotted chowing down at a low-key Mexican chain Friday with his girlfriend and the rest of his fam in El Centro, CA ... almost 2 hours east of San Diego.
The restaurant of their choice ... La Resaca -- known for their seafood and with a nearly 5 star rating off a ton of good reviews.