Why does food cooked in the microwave heat inside-out?
Your microwave isn't exactly cooking your food from the inside out. Instead, what is happening is that some parts of your food that happen to be on the interior are being heated faster than those parts of the exterior that you observe. This sort of uneven heating is intrinsic with how microwaves work.


  Microwave ovens cook unevenly because a pattern of standing waves forms inside the oven chamber, and the pattern creates an array of hotspots throughout the oven's volume. An operating frequency of 2.45 GHz will produce a wavelength of around 12.25 cm, and the regions of maximum intensity (hotspots) will be at half-wave points, or every 6.125 cm, but in a complex 3D pattern


From here

So, in your microwave the waves happen to be arranged just so the inside of your casserole heats up faster than the outside. 

In microwave ovens that do not have a rotating carousel, this effect is even more pronounced, although even using a carousel is not a panacea. Many microwave oven recipes often include instructions to rotate and/or stir the food intermittently as a means of distributing the heat more evenly for the same reasons; changing the position of the food within the oven has the effect of shifting the hotspots around in the food as it cooks.

So, my suggestion is to try changing the position of the food in the oven over time, particularly if it's being heated over several minutes, as that should help it heat more evenly. If you can, it is also a good idea to stir the food intermittently as well. Personally, after the microwave finishes the allotted time, I also will stir and then wait a few minutes before serving to let the hot spots naturally dissipate a bit via conduction.