It is very important to test first.... the test is to let you know that your cat is at an UNFED number that is safe to shoot. No matter what insulin you are using, you test first, then once you decide SHOOT or NO SHOOT, you can feed.
You can prepare the food, have it all ready in a dish; that's no problem.
Test your cat and decide if the BG number is safe for shooting. if the number is not safe, if you are delaying a shot to see if the number is rising, giving food before knowing where the cat's numbers are going would skew the next number because of course it would be a higher number..... because of the food.
For an insulin which requires food in the cat before the insulin is given, you can give the cat the prepared food after you have decided that you are going to give the shot. Test and number is safe, then give food and wait your 15min or 30min to give your insulin shot.
For insulins like Lantus and Lev, the cat does not have to eat before getting a shot, so just test first, and put down the food ONLY after deciding if you are going to give a shot or not.
Too bad for hungry cats; you need to know that it's safe to give insulin. If you feed, then your test is food skewed, you may shoot based on that temporarily raised number..... numbers start dropping when the food wears off in an hour or two, and then the insulin kicks in shortly after that which results in even lower numbers, and you got yourself a hypo possibility.
Test first and then decide shoot or no shoot. After that decision, do whatever you like for sequence in feeding and shooting.