Heat the olive oil in a large, heavy skillet over medium-high heat. Meanwhile, combine the chili powder, cumin, and smoked paprika in a small bowl.
Rub the spice mix into the beef, covering each side evenly. Once the oil is hot (it will shimmer a bit in the skillet), place the beef in the skillet and sear on each side.
Remove the beef from the skillet and place in the bottom of a slow cooker. Leave the pan on the heat, and add in the beef stock to deglaze, scraping up any browned bits from the bottom of the pan. Add the tomato paste and minced chipotle and whisk into the pan sauce. Bring the sauce to a boil, reduce the heat, and simmer for a few minutes until the sauce has reduced and thickened slightly.
Add the diced onion and minced garlic on top of the seared beef in the slow cooker. Pour the pan sauce down over the onions, garlic, and beef. Cover and cook on low for 6-8 hours.
Remove the beef from the slow cooker and shred with two forks. Use a slotted spoon to remove the onions from the slow cooker, and mix into the shredded beef.
Serve the beef in the flour tortillas, topped with your favorite taco toppings.
Some topping ideas include:
* guacamole or avocado
* fresh salsa
* Mexican rice
* grated cheese
* fresh lettuce
* squeeze of lime juice
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