There is a chance, however slight, that entering this patio through the restaurant will lead to an insurance agent ready to write policies protecting businesses against the loss of gifted wait staff. But let's assume that this establishment really wants to tell patrons that entering the patio through the restaurant is the best way to make sure the help knows you'll be consuming adult beverages and hot wings outside.
Then this sign should substitute ensure for insure.
Every reference book will tell you the same thing, but let's refer to the AP Stylebook since Mr. Rewrite's students ignore it at their peril.
Ensure means to guarantee. For example: Having an AP Stylebook on the desk is a way for students to ensure that Mr. Rewrite won't whack their knuckles with a ruler. And then there's Ensure, a brand of nutrition drink marketed to those only slightly older than Mr. Rewrite -- cough.
Insure applies to insurance. For example: His employer took out liability coverage to insure against the possibility that Mr. Rewrite really would whack students' knuckles with a ruler.
Assure, meanwhile, means to make sure or give confidence. For example: If Mr. Rewrite's boss reads this post he'll probably have to assure her that he wouldn't really whack students' knuckles with a ruler.
So would Mr. Rewrite really whack students' knuckles with a ruler? He won't tell. The only way to ensure that he doesn't is having a stylebook in plain sight at all times.