How to Make It
In a small bowl, mash the bouillon cube into 2 teaspoons of hot water until dissolved. Whisk in the mayonnaise and Sriracha and season with salt and pepper.
In a medium skillet, heat the oil. Add the onion, cover and cook over moderate heat, stirring occasionally, until golden brown and softened, about 10 minutes. Stir in the vinegar and season with salt and pepper.
Meanwhile, in another skillet, cook the bacon over moderate heat, turning once, until crisp, 6 minutes. Drain on paper towels.
Set a griddle over 2 burners and turn the heat to moderate. Butter the bread slices on 1 side. Spread the other side of 4 bread slices with the spicy mayonnaise and top with the onion, cheddar, cheese curds and bacon. Close the sandwiches buttered side out and griddle over moderately low heat until crisp, about 4 minutes per side. Cut the sandwiches in half and serve.
Cheese curds are available at specialty food shops and cheese shops.