In a medium size pan add the refried beans, cumin, garlic, pepper, and water. Stir together and warm on low heat.
For the Tortillas:
Cut each corn tortilla into the shape of a heart. Heat a small skillet on medium heat and add the canola oil. To test if the oil is hot enough dip just an edge of the tortilla in the oil and if it sizzles add the entire heart tortilla. Cook until each side of the tortilla is golden brown (about 3 or 4 minutes). Remove the tortilla from the oil and set on a paper towel-lined plate. Repeat until all tortillas are cooked. Set aside.
For the Tostadas:
Spread the warm refried beans on top of the tortilla shells. Next spread the salsa on top of the beans and top with the cheese, purple cabbage, tomatoes, cilantro, green onions, and lettuce (optional).
Add the sour cream to a pastry bag with your favorite decorative tip. Pipe the sour cream around the outside edges of the tostada so that it resembles the look of lace. Or make any decoration with the sour cream you would like. Serve warm or at room temperature as an appetizer.