Set a heavy Dutch oven over medium-high heat, add a drizzle of oil and brown the ribs on all sides, sprinkling with salt.
Add the chili powder and cumin and cook for another minute.
Add the beef stock and tomatoes, cover and braise at 300˚F for 3-3 1/2 hours, or until the meat is very tender. Shred the meat from the bones.
In a small skillet, heat about half an inch of oil until hot, but not smoking; cook each corn tortilla for a minute or two, until golden and crisp.
Transfer to a paper towel-lined plate.
Top each crispy tostada with some pulled beef, cheese, avocado and salsa.