Heat the oil in a large deep skillet or pot over medium-high heat. Season the brisket generously with black pepper. Place in the pan and cook until the surface is a rich brown color, not burnt, but dark. Lift the roast and scatter the onions in the pan. Place the uncooked side of the roast down onto the onions. Repeat the browning process.
Add the garlic to the pan, and fill with enough water to almost cover the roast. Bring to a simmer. Reduce heat to low and cover with a lid or tight-fitting aluminum foil. Simmer for 4 hours, turning the roast over once halfway through. The roast should be fork tender.
Remove the brisket to a serving platter. Bring the broth in the pan to a simmer, scraping the bottom to loosen any browned bits. Cook until reduced to a thin gravy. Taste and season with salt and pepper if needed.