Looking for steaks with casual, younger crowd in mid town
Obviously both Artisanal and Keen's are better, fancier places. But if you're looking for a lower price point and a reasonably decent meal, you may actually want to consider Tir Na Nog (33rd and 8th) If you get a table away from all of the after-work happy hour hubbub, it's okay. The food's better than you'd expect. I don't see french onion soup on their menu, but the apps there are worth trying. The after work crowd is more "commuter" than young.
... but it's only steps away from the garden.
Don't know about the french-onion soup requirement but Keens Steakhouse (72 W. 36th St) is near MSG and has a great typical steakhouse menu. Even closer to MSG is Nick & Stef's (9 Penn Plaza) which is fine but IMO not in the same league as Keens.
Nick & Stef's