I've had good steaks at Jade Garden, and I'm told Mambo at Mata Chica has some excellent steak, beef imported from Honduras, though I haven't tried the steak there myself.
The Running W restaurant at San Ignacio Hotel has pretty good steaks, Robert's Grove in Placencia has good imported beef, and there's a new steak house at the Tourist Village in Belize City.
None of these places will give Ruth's Chris, Morton's or Spark's a run for their money, though. As noted, most beef from Belize (and from elsewhere in Central America) is grass-fed and without much marbling. It's better for you than corn-fed, aged U.S. Prime, but it sure doesn't taste as good.
Belize First www.belizefirst.com