Add oil and flour to a large saucepan on medium heat and whisk to combine. Add spices and whisk together before adding tomato purée and stock. Bring to a boil and allow to cook for 15 minutes. Remove from heat.
Roughly cut steak into palm-sized pieces and add to slow cooker along with sauce, making sure the meat is fully submerged. Slow cook for 6 hours.
Remove beef from slow cooker and shred using two forks. Place in a serving bowl with a ladle of sauce. Reserve remaining sauce to serve if desired.
For the Guacamole
Place avocado and lime juice in a bowl and mash with a fork until well combined and smooth. Season to taste with salt and pepper and set aside.
For the Salsa
Combine tomatoes, onion and lime juice in a bowl and season to taste with salt and pepper. Stir to combine and set aside.
To assemble, place meat, guacamole and salsa in tortillas and top with sour cream. Serve with extra sauce for dipping, if desired. Enjoy.
Notes
If you would like some spicier tacos you can add more cayenne pepper. If you would like milder tacos, you can omit it altogether. Add as much or as little as you like!
If you can't get round steak, you can use any cheap cut of beef. Try chuck steak, beef cheeks or blade steak.
This recipe can also be pressure cooked for 1 hour to save time! It's just as delicious!