Coat cubed beef in flour, shaking off excess. Set aside.
Remove the tops of the pearl onions and blanch in boiling water for 30 seconds.
Drop onions into an ice bath and peel. Set onions aside.
Heat oil in a cast iron pan over high heat.
When hot, sear beef in batches, do not overcrowd the pan.
Set seared beef aside.
Sear onions in the same pan until lightly golden brown.
Deglaze the pan with red wine.
Add rosemary and reduce red wine by half.
Add demi-glace and reduce by 1/3.
Remove rosemary and add beef and onions back into the sauce, simmer.
While the beef is simmering, blanch the rapini in a pot of boiling, salted water, for two to three minutes, until crisp-tender; remove and drain. Keep warm.
Remove beef and sauce from heat, swirl in cold butter to emulsify, do not overheat. Adjust seasoning.
Transfer the beef, onions, and sauce to a dish and serve with the warm rapini.