While you really shouldn't "cook" a raw brat straight from the freezer, if you don't have 25 minutes to let the brats thaw in tap water, you can. However, you will need to break an unwritten bratwurst cooking rule to see if they are done; you'll have to poke them with a meat thermometer and allow juices to seep out.
On Stove Top
Place a pot of water on the stove top.
Turn the burner on high and bring the water rolling boil.
Drop the frozen brats in the water.
Cook for 14 to 16 minutes, two to four minutes longer than if you thawed them first.
Insert a food thermometer into the center of one brat to check doneness for safe cooking preparation. The internal temperature should read 165 degrees Fahrenheit.
Microwave Quick Start
Set the microwave on medium-high power.
Place the brats on a plate in the center of the microwave. Make sure the plate is microwave safe.
Set the timer for two minutes. Cook the brats; this mode is technically the defrost mode, but can start to cook food if left in too long.
Remove the brats and place on a hot grill, cooking for another eight to 10 minutes. Turn several times to make sure all areas are being heated and cooked evenly.
Insert a meat thermometer into the center of the brat to make sure it is cooked to at least 165 degrees Fahrenheit for food safety.