Heat oil to 375 degrees F in a deep-fryer or a large, heavy Dutch oven (oil should come no more than halfway up the sides of the pan).
Combine flour and ranch dressing mix in a shallow bowl. Set aside.
Cut about 1/2 inch off the tops of the onions and peel them. Cut 12 vertical slices until you almost reach the bottom of the onion (do not cut all the way through). Remove the center of each onion. Don't throw away!
In a second bowl, mix tempura batter with chilled soda water. Don't over-stir; batter should be slightly lumpy. Dredge each onion in seasoned flour, shaking to remove excess flour. Dip in tempura batter and make sure to spread "petals" apart to get an even coating of batter.
Carefully place 1 onion in fryer. Fry for about 2 to 3 minutes or until golden. Remove and carefully ensure that petals are separating, using a utensil to help spread them if necessary. Recoat onion with batter and return to oil for about another 5 minutes. Remove from oil and drain on paper towels. Repeat with remaining onions.
To make the dip, mix all ingredients in a bowl, adding hot sauce to taste. Serve with onion blooms.