In bowl, toss Brie and cream cheese with flour to coat. Set aside.
Place 8 or 10 in. (20 or 25 cm) cast-iron skillet on grill preheated to medium. Add butter. Stir in onion, garlic and salt; cook until onion is softened but not browned, about 2 min. Pour in beer; bring to a simmer (avoid rapid boil).
Add cheese mixture to skillet in 3 portions, mixing to melt each addition before adding next. Don’t worry if some small lumps remain. Stir in hot sauce, if using. Move skillet to indirect heat or off the grill. Garnish with parsley and smoked paprika. Serve with grilled plain or garlic bread for dipping.
Alternatively, make dip on stovetop on medium heat.
Use leftover dip as a sauce on chicken, grilled veggies. Or toss with hot pasta.