Heat up stock and stir in dry herbs, salt, red pepper flakes, and sugar.
Place rump roast into the slow cooker and poke some holes all over the roast using a small knife, about 1 1/2 inches deep. Peel garlic cloves and stick individual garlic cloves into the holes.
Slice onion and mushrooms, not to thin, and add them into the slow cooker.
Pour prepared stock mixture all over the beef and veggies.
Close the lid and cook on low for 8-10 hours.
Take the beef out once it's done and shred it using two forks.
Take onions and mushrooms out of the liquid using a slotted spoon and mix them in with shredded beef.