To make mustard, mix dry mustard, 1/3 cup beer, and horse radish together in a small bowl.
Let mustard stand for 15 minutes.
Stir the mustard before serving.
If mustard is too thick, add beer by the tablespoon until desired consistency is obtained.
Cut bratwurst in half horizontally.
Pour remaining beer into a bowl.
Marinate bratwurst in beer for 1 hour.
Drain and discard marinade.
Cook bratwurst, cut side down, for 2 minutes over medium-high heat.
Turn bratwurst over and continue grilling for 2 minutes or until cooked through.
Remove from grill.
Warm rolls, cut side down, on the grill.