Remove the brisket from the refrigerator 30 minutes prior to cooking, allowing it to reach room temperature. Coat the brisket with olive oil, ensuring an even layer surrounds the meat. Generously season with salt, black pepper, dried thyme, and rosemary, pressing the spices into the surface.

In a medium saucepan, heat 1 tablespoon of olive oil over medium heat. Add the diced yellow onion, diced shallot, and minced garlic, sautéing for about 5 minutes until they are soft and fragrant. Incorporate the cranberry jelly, Worcestershire sauce, and Guinness into the mixture, stirring well. Allow the sauce to simmer for approximately 10 minutes, thickening slightly and blending the flavors together beautifully.

Preheat your oven to 250°F (120°C). In a large roasting pan, pour half of the prepared sauce, spreading it evenly across the bottom. Layer the sliced onions on top, followed by the trimmed brisket, fat-side down. Pour the remaining sauce over the brisket, ensuring it's fully coated. Arrange the red potatoes and dried cranberries around the brisket, followed by sprigs of fresh rosemary and thyme for added aroma.

Cover the roasting pan tightly with aluminum foil to trap moisture. Place the pan in the preheated oven and let the brisket roast for approximately 4 hours. Check for doneness towards the end; the meat should be fork-tender and infused with the rich flavors of the sauce.

Once the cooking time is complete, carefully remove the brisket from the oven, lifting the foil to avoid steam burns. Allow the meat to rest for 30 minutes before slicing; this step locks in the juices for maximum tenderness. Serve your beautifully sliced brisket with the roasted potatoes and a generous spoonful of the pan sauce drizzled over each portion.