Season the rump roast with salt and pepper on all sides. In a large skillet, heat the olive oil over medium-high heat. Sear the rump roast on all sides until it's browned, about 3-4 minutes per side.
Place the sliced onion, baby carrots, baby potatoes, garlic, and bay leaves at the bottom of the slow cooker. Add the seared rump roast on top of the vegetables. Sprinkle the thyme and rosemary over the roast. Pour the beef stock around the roast.
Cover and cook on low for 8 hours or on high for 4-6 hours, until the meat is tender.
Once the roast is done, remove the roast and vegetables from the slow cooker and transfer them to a serving platter. Cover to keep warm.
Into a sauce pan, melt the butter over medium heat. Stir in the flour until there are no lumps. Cook the butter mixture for another minute.
Strain 3 cups of the cooking liquid into the saucepan with the butter mixture and whisk to combine. Continue whisking until the liquid begins to simmer and thickens. Adjust and season with salt and pepper to taste.
Slice or shred the rump roast and serve with the vegetables, topped with the gravy. Garnish with chopped fresh rosemary if desired. Enjoy!