1 very large onion
Peel the onion and slice off the top. Do not slice off the root end. Using a small, sharp knife, remove the center of the onion down to the root (but not including the root). You can use an apple corer to help do this. Using a sharp knife, slice from the top down to the root, but not through the root, all around the onion. The slices should be fairly small but not tiny.
Soak the onion in some ice water to make it open up. Use your fingers to gently open the "petals". Drain the onion.
Place the flower in a bowl and add some Cajun seasoning to taste. Mix well. Dip the onion in the flour to coat, or shake onion & flour gently in a plastic bag.
Mix the milk and egg, and add Cajun seasoning if you'd like. Dip the onion in it to coat, and then back into the flour.
Heat the oil to at least 350 degrees. Place the onion in the oil, root side down. Try to spin it as you go. Cook the onion until it's nice and brown and soft. Remove it from the oil and drain.
To make the dipping sauce, mix sour cream, mayonnaise, Cajun seasoning, cayenne pepper and chili powder to taste. If desired, place the sauce in the center of the onion blossom.