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The slit should go from the exterior of the orange to the center of the orange. This will create a triangular section of orange.
Take an orange and remove the peel and the white pith with a chef's knife. Cut a slit into the orange using a chef's knife. Cut another slit about 1/4" to the right of the first one. Remove the section and place it in a medium bowl. Continue cutting out triangular orange sections until the entire orange is sectioned. Combine the red onion, jalapeno and cilantro with the oranges. Taste a spoonful of the mixture and add salt a little bit at a time until the salsa tastes right to you.