The best grilled bratwurst

I love bratwurst. Grilled is good, but here's how to make it great:

Place the brats in a cast iron skillet. Add beer, mustard, chopped onions.

Bring to a boil, reduce heat, simmer 20 mins.

Remove from the liquid; Grill on the stovetop, on a preheated cast iron grill pan.

Transfer grill pan to the oven, finish off at 300, so they cook all the way through without charring.