i've had the solomillo too (it's called solomillo de novillo on the Spanish menu - I've had it twice over the last month!) - which is steer meet (young, castrated bovine). Meat cuts do not translate well from Spanish to English - particularly not on Iberia menus which appear to be Google translated.
'Beef sirloin' would be a perfectly adequate translation, but the Spanish market identifies the meat by age much more than in the UK/US. The frequent mistake is when people translate ternera to be veal - when actually it is just young beef and red rather than pale coloured (ternera lechal is veal).
I agree it was tasty and well cooked!