Make the curtido: Place chopped cabbage in a large heatproof bowl and cover with boiling water for 1 minute. Drain and return to bowl. Add 1/4-cup cold water, carrots, bell pepper, red onion, vinegar, salt, and oregano Add beets and horseradish if desired. Refrigerate for at least 2 hours.
Make the pupusas: Prepare tortilla dough according to package using 1/2-teaspoon kosher salt and 1 1/4 cups water. Mix thoroughly to form very soft dough, adding water in small doses if too dry. Form dough into 25 small balls and flatten each until about 1/2 inch thick.
In a medium bowl, combine chicharrón, queso fresco, and refried beans and place a small spoonful in center of each round. Fold edges of masa up over filling and press edges together to seal. Lightly pat balls until flattened to about 1/2 inch thick.
Heat griddle or heavy skillet until very hot. Lightly brush skillet with cooking oil (or coat with cooking spray). Cook pupusas until lightly browned, about 5 minutes on each side. Serve pupusas with curtido.