Sardines are canned, which means the high heat of a pressure canner. That is what softens the bones. If you are wanting softer bones for other kinds of fish, you can either cook them using a pressure cooker or can them. However, this will only work for smaller fish that have smaller bones.
If your main concern is that you will swallow a bone and choke, then buy your fish as fillets only. Most fish fillets have no bones at all. The pin bones that are in salmon fillets can be removed with needle nose pliers, and are so small that you couldn't choke on them if you tried.
I agree with Joe: wet methods are generally good for preventing browning.
The general policy is just to use temperatures which are as low as possible (while ensuring food safety and cooking until "done"). The other thing you want to avoid is very long cooking, since browning reactions can still occur if you cook something long enough at a low temperature.
Wet methods are often best both because they naturally keep the temperature of the food below boiling, and the circulating liquid helps to raise the food's temperature faster (which tends to allow quicker cooking). Compare that to roasting (or similar techniques) at a very low temperature -- in that case, it will often take much longer for the food to get up to a "done" temperature (which can sometimes be a food safety issue, particularly in large hunks of meat).
Acids can also inhibit browning a bit, though low temperature and shorter time are most important.
The sous vide technique is perhaps the optimal method for preventing browning, since you cook the food in a sealed bag to a low temperature (and usually one specified to ensure food safety). You also contain all the juices, etc. of the meat, rather than other wet methods where they tend to be lost to the surrounding liquid. (If you are going to consume the liquid as well, that can be good, though.)
The main problem with all of these techniques is that some meats are considered much less appetizing without any browning. For example, many people love a steak cooked to a perfect "medium rare" inside, but they brown the exterior -- and even that thin layer of browned bits is enough to give a lot of flavor to a bite of even a thick steak. If you didn't brown the outside and just had the whole thing "medium rare," it would be a different experience. Think of the interior of a medium rare prime rib -- in that case, many people often tend to use some salty flavorful "jus" or another kind of sauce to add flavor while eating the meat. So in some cases you might look for recipes that add a sauce or other complementary flavorful foods to go with the meat (and replace the missing flavor from browning).
Best Answer
Sprat and even larger fish like small jack mackerels (up to 10 cm length) are typically eaten with their spine bones when fried. The fins and heads are removed though.