Layer the thinly sliced onion into the bottom of the slow cooker insert.
Place the bratwurst on top of the onions in an even layer.
In a 2 or 4 cup capacity (microwave safe) measuring cup, melt the butter in the microwave. Add the flour. Microwave in four, 30-second increments, stirring with a fork each time. The flour will bubble up and the mixture will get a deeper yellow color.
Add the Dijon mustard, sugar, garlic powder and salt to the butter mixture. Stir well to combine, until smooth.
Slowly pour in the beer, be careful as it will bubble up. Stir to combine. Pour the beer sauce evenly over top the onions and brats. Nestle the bay leaf into the sauce as well.
Cook on high power for 4-6 hours, or until the brats reach a minimum internal temperature of 160 degrees F. And the brats have started to brown on the outside.