Yu Darvish -- who's in a $126 MILLION contract -- completed his first minor league rehab start this week ... and celebrated by spoiling BOTH TEAMS with a very expensive Ruth's Chris steak dinner.

The Chicago Cubs had sent the pitcher down to the Class A Affiliate South Bend Cubs to help him work his way back from a triceps injury ... and, Monday night, he took the mound with a bunch of guys who make WAY less cash.

In fact, most of the players only make around $45k per year (and that's if they're playing consistently for the entire year).

So, Yu -- being the generous rich dude that he is -- pulled out the checkbook and bought a massive spread of steak, salmon, sides and some bomb ass cookies for his team and the opposing West Michigan Whitecaps.

Total bill for it all had to be pricey ... a steak from the fancy spot runs about $50 a pop, while the fish costs about $30 a piece.

As for the minor leaguers ... Whitecaps reliever Max Green took to Twitter to thank Yu for the meal afterward.