Begin by slicing the onions into thick, even rings; this ensures they cook uniformly in the slow cooker. Aim for about half an inch in thickness for optimal texture.

In a medium skillet over medium heat, melt two tablespoons of butter until bubbly. Add the sliced onions and sauté them for approximately 5 minutes, stirring regularly, until they are just softened and translucent but not caramelized.

While the onions are sautéing, grab a mixing bowl and combine the sweet barbecue sauce, Dijon mustard, and hot sauce. Whisk the mixture thoroughly until smooth and well blended.

Transfer the sautéed onions into the bottom of your slow cooker, spreading them evenly. Pour the barbecue mixture over the onions and gently stir to coat them well.

Preheat your grill or cast iron skillet over medium-high heat. Add the brats and grill them for about 3 minutes on each side until a nice golden-brown crust forms.

Carefully place the grilled brats on top of the onion mixture in the slow cooker. Pour in the two bottles of beer, allowing it to envelop the brats and onions. Cover and cook on low for 4 to 6 hours.

Once cooked, the Slow Cooker Beer Brats with Onions will be deliciously aromatic. Serve them warm on toasted buns, heaping spoonfuls of the savory beer-onion sauce on top.