Preheat oven to 375°F (190°C). Arrange oven rack to the middle of your oven.: As you warm the oven you will likely notice a neutral, dry heat settling in the cavity that signals readiness. Ensuring the rack is in the center helps the heat hit the skillet evenly, which prevents one side from browning more quickly than the other. If the oven is not at the correct temperature the dip may take longer to bubble and the texture could be slightly gummy rather than luxuriously creamy. A common mistake is relying on the oven light alone, so I recommend using an oven thermometer for accuracy.
In an 8-inch cast-iron skillet (or an ovenproof pan), melt the butter over medium-high heat. Sauté garlic until fragrant (30 seconds). Add the chicken and hot sauce and simmer until sauce has thickened and reduced by half (about 2 minutes).: Right away you will smell the butter warming and then the garlic will release a warm, nutty scent as it sautés for just seconds. When you add the shredded chicken and Frank's Original Red Hot Sauce the aroma shifts to tangy and slightly vinegary. Simmering lets excess moisture evaporate so the sauce clings to the chicken , avoiding a watery dip. If you skip the reduction you may end up with a runny mixture that dilutes flavor; stir frequently to prevent the garlic from burning, which causes bitterness.
Reduce heat to low and stir in cream cheese; mix until combined. Take off heat, stir through sour cream and top with both cheddar cheeses over the top.: When you add softened cream cheese the pan will go from glossy to luxuriously thick and creamy. Use a spatula to coax the block into the sauce until it is silky, then the chilled sour cream will cool and brighten the mix. Placing the shredded cheddar cheese on top rather than stirring it all in creates a melty, gratin style surface after baking. Overheating the cream cheese can split the texture, so keep the heat gentle and remove promptly from the burner to maintain that smooth mouthfeel.
Bake until bubbling around the edges and the cheese has melted (about 10 minutes). Broil (or grill) for a further minute to brown on top.: As the skillet heats in the oven you will hear a gentle simmer and see tiny bubbles forming at the edges, a visual cue that the dip is hot and cohesive. The cheeses will melt into a glossy blanket, and broiling briefly adds a toasty, golden color and slight textural contrast on top. Watch the broiler closely because it can go from perfectly browned to burned in seconds, so I tend to hold the oven door slightly ajar and stand nearby. Avoid leaving it unattended during broiling to prevent bitter char from developing.
Immediately garnish with blue cheese and green onions. Serve with vegetable sticks, chips, crusty bread pieces for dipping.: Right after the dip comes out you will notice steam rising and the surface shimmering, which is the ideal moment to add the crumbled blue cheese and sliced green onions . The cool, pungent blue cheese creates sharp flavor contrast, while the green onions add fresh aroma and a crisp pop. Serving hot ensures the textures remain gooey and satisfying; letting it sit too long will thicken the dip, so serve promptly. If the dip firms up, a short return to the oven or microwave will revive the melt.