Heat oil in a pan over medium high heat. Season beef all over with salt and pepper then brown well on all sides. Remove and set aside.
In the same pan, cook the onions until softened and lightly browned, about 5 minutes. Add garlic and cook 1 minute.
Transfer the onions and garlic to the slow cooker, then place the beef on top.
Mix hot water, stock powder or cubes, Worcestershire sauce, balsamic vinegar, and tomato paste in a jug. Pour over the beef and tuck the fresh herbs around it or if using dried, simply sprinkle over the top.
Cover and cook on low for 8 to 9 hours or high for 5 to 6 hours or until the beef is cooked and tender. Remove the beef and set aside to rest whilst preparing the gravy.
To make the gravy, strain the cooking liquid from the slow cooker into a saucepan, pressing the onions to extract flavour. Bring to a rapid boil and cook for 5 minutes to slightly reduce.
In a jug, whisk the gravy powder with ⅓ cup hot water until smooth. Slowly pour into the boiling liquid, whisking constantly, until the gravy thickens. Simmer for 1 to 2 minutes, then remove from heat.TIP: If the gravy is thinner than you like, simmer for a few more minutes or add a little extra slurry. Slice the beef and serve with gravy and your choice of sides.