Place the beer, caraway seeds, mustard seeds, onion, garlic, and bratwurst in a stockpot and allow to marinate for 1 hour.
Bring the pot to a simmer over high heat and cook for 7 minutes, reducing heat as necessary to just maintain the simmer.
Remove from the heat, and let sit for another 30 minutes.
Remove the brats from the liquid.
Brush with avocado oil.
Ninja Foodi Grill
Preheat your Ninja Foodi Grill on the Grill setting to high heat and set the timer for 10 minutes. Cook for 4 to 5 minutes per side, until lightly charred and heated through.
Serve warm.
Outdoor grill
Heat your grill to high heat.
Cook the Brats for 8 to 12 minutes, until the casing are golden brown.