In a large heavy-bottomed pot, melt the butter over high heat. While butter is melting, generously season entire brisket with 2 tsp of the salt and pepper. Place the brisket in the hot pan (make sure that your oven vent is turned on) and give each side a good sear, until a nice brown char develops on both sides (about 5 minutes on each side should suffice). Place the seared brisket in the slow cooker.
With the same pan you used to sear the meat, add the onions and the remaining ½ tsp salt and sauté over medium heat until the onions are wilted and start to brown (10 to 15 minutes). Transfer the onions to the slow cooker over the brisket. Add the garlic, broth, and coconut aminos to the slow cooker.
Cover and cook the brisket on low for 8 hours. Slice or shred the meat, season with more salt as needed, and serve warm, or place the entire pot in the refrigerator to chill overnight—this method will allow you to skim off the excess fat that cooked away from the brisket.
To reheat the brisket, place the meat (thawed, if previously frozen) on a baking dish and bake (with the onions and a few ladles of the remaining liquid) in a preheated 350°F oven for about 30 minutes.