Heat the oven to 350°F. Brown the beef in a skillet over medium-high heat for 8 minutes, crumbling it and cooking it to 160°F.
Stir in cumin, paprika, oregano, salt and tomato sauce. Simmer for 3 minutes, until the sauce clings to the meat.
Heat the beans and water in a covered saucepan over low heat, stirring occasionally. Warm the tostada shells in the oven for 3 to 4 minutes.
Mix the sour cream and lime juice. Spread each warm shell with beans, then add beef, cabbage, a little lime cream and queso fresco.
Serve immediately, two tostadas per person, while the bases still snap when bitten.