Place cabbage in a large bowl; sprinkle salt and red pepper powder on top. Toss well and let stand until cabbage looks wilted, about 1 hour.
Bring water and rice flour to a boil in a large pot. Simmer for 5 minutes. Remove from heat and let cool, 10 to 15 minutes.
Combine onion, Asian pear, and green onions in a half-gallon jar. Add wilted cabbage, garlic, ginger, and sweetener. Pour water mixture on top. Seal jar and keep at room temperature to ferment for 1 to 2 days. Place in the refrigerator.