2 large yellow onions, peeled and sliced into 1/3-inch rings
2 4-inch cinnamon sticks
5–6 Kaffir lime leaves (optional but recommended – you can find them fresh or frozen at many Asian grocery stores and some mainstream supermarkets.
Make the Spice Paste: Put the chilis, shallots, garlic, ginger, lemongrass, turmeric, salt and cloves into the bowl of a food processor with the chopping blade attached. Add 4 tablespoons of the thick coconut cream from the top of the can of coconut milk. Pulse everything until you get smooth, thick paste, in which you can still make out the texture of finely minced peppers.
Brown and Prepare the Brisket: Preheat oven to 350ºF. Dust the brisket with flour or matzo meal, if using. (it helps with browning but is not essential) Heat 2 tablespoons of oil over medium high heat, in a large heavy pot or Dutch oven, with a lid. Add the brisket and brown it for about 5 minutes per side. It won’t be uniformly brown but there should be some nice brown spots over the surface. Transfer the meat to a plate. Add another tablespoon of oil to the pot and spread it out with a spatula. Add the spice paste and cook it, stirring often, for 4-5 minutes, until it starts to brown. Transfer the paste to a bowl. Add the final teaspoon of oil to the pot with the sliced onions. Cook, stirring often, for about 5 minutes, until the onions have softened a bit. Pour the remaining coconut milk over the onions and stir. Place the brisket on top of the onions. Coat the top and sides of the brisket with the spice paste, using the back of a spoon to smooth it out evenly. Scatter the lime leaves and cinnamon sticks around. Cover the pot and cook it in the center of the oven for 30 minutes.
Braise and Baste: Turn the oven temperature down to 325ºF. Gently baste the brisket with some of the coconut sauce. Return it to the oven for 2 hours, basting every hour.
Remove the pot from the oven but leave the oven on. Gently transfer the brisket to a cutting board and, with a sharp knife, slice it thinly (1/8-inch) across the grain. Transfer the sliced meat back into the pot. If you can, keep it in it’s original shape. Baste it, separating the slices a bit so the sauce gets in between. Cover the pot and return it to the oven for a final half hour of cooking.
Serve hot. It reheats really well and freezes really well. Defrost and reheat in a 325º oven, covered, for 45 minutes or so.