Preheat oven to 325°F (165°C). Season brisket generously with salt and pepper.
Heat olive oil in a large Dutch oven over medium-high heat. Sear brisket 4–5 minutes per side, until deeply browned. Remove and set aside.
Add onions to the pot, cooking 10–12 minutes, until deeply caramelized. Add garlic and cook 1 minute more.
Stir in broth, tomato paste, and sugar substitute, scraping up any browned bits.
Return brisket to the pot, add bay leaves, cover, and transfer to the oven.
Braise 3 to 3 1/2 hours, until fork-tender.
Rest 15 minutes, then slice against the grain and serve with the onion gravy.