Preheat oven to 300°F with the rack located in the lower middle.
In a large bowl, combine cubed beef, chili powder, and salt. Toss well.
Melt fat over medium heat in a large, oven-proof dutch oven. Add the onions and sauté until translucent.
Stir in tomato paste and garlic, and cook for 30 seconds.
Add the seasoned beef, and pour in the salsa, stock, and fish sauce. Bring to a boil.
Cover the pot and place it in oven for 3 hours or until the beef is tender.
Taste for seasoning and adjust if necessary. (At this point, you can store the beef in the fridge for up to 4 days or freeze for up to 3 months and reheat right before serving.)
Spoon the beef onto a serving platter and top with cilantro and/or radishes if you’ve got ’em.
Notes
Assemble your own tacos with lettuce leaves, guacamole, diced onions, and cilantro.