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.
Check seasonings and adjust to taste.