Fill a 6-quart pot 2/3 full of water; add the crab boil and mix until blended. Cut the potatoes in half and slice into 3/8-inch thick pieces. (This can be done ahead of time; cover the potatoes with ice water until ready to cook.) Add the potatoes to the pot. Bring water to a boil and cook the potatoes until fork-tender.
While the potatoes are cooking: in a medium saute pan on medium heat, cook bacon until crispy; remove and drain on a paper towel. Add red onion to bacon grease; cook until caramelized. Just before onions are done, add garlic and cook until lightly brown.
In a bowl, place butter, 1/2 the cheese, 1/2 the bacon bits, 1/2 the green onions, the cooked red onions and garlic, cayenne, paprika, salt, pepper, and sour cream; mix together thoroughly.
Strain potatoes and add to bowl; let stand for a few minutes or until cheese starts to melt. Fold ingredients together, trying not to break up the potatoes excessively. Top with remaining cheese, bacon, and green onions.