First, make your caramelized onions: Heat the butter in a large skillet over medium heat until melted. Add in the onions and cook until soft and translucent, about 1-2 minutes. Season with a pinch of Belize spice and salt. Reduce the heat to medium-low\/low and continue cooking, stirring occasionally, until well caramelized and softened, about 15 minutes more. If dry, add in a splash of water. \nNext, make your pimento cheese: Paddle the cream cheese and Belize spice in the bowl of an electric mixer until completely smooth. Add in the cheddar cheese, mayo and pimentos and season with a pinch of salt. Continue beating until well combined. Chill until ready to assemble. (Any leftover pimento cheese makes a great dip for chips!). \nPreheat a grill, grill-pan or large skillet to medium-high\/high heat. Form the ground beef into 4 equal-sized patties, about 1-inch thick. Season both sides of each patty with Belize spice and salt. Grill burgers until golden brown and slightly charred on each side, about 4 minutes per side, or until cooked through to desired doneness. \nStir 1 teaspoon Belize spice into your remaining ¼ cup mayo. \nSpread Belize mayo over the bottom bun and pimento cheese on top. Top with a burger, lettuce, tomato and caramelized onions. Sandwich together and dig in!