Brine at least 12 hours or more before you intend to cook. Put the salt, sugar, garlic, peppercorns, ground mustard, and onion powder in a non-metal container. Pour warm water over it all. Stir so salt and sugar dissolve. Cool. Add cornish game hens, marinate until ready to cook. Preheat oven to 425 degrees. Roast at 425 degrees for 25 minutes. Turn down the temperature to 400 degrees and roast for another 20 minutes. Finally, turn down the temperature to 375 degrees and roast another 25-30 minutes.