Place the softened cream cheese, freshly grated sharp cheddar, freshly grated Monterey Jack, and mayonnaise directly into a large ceramic mixing bowl (or the bowl of a food processor for a smoother texture).

Using a heavy wooden spoon or an electric mixer, vigorously beat the cheeses and mayonnaise together until the dairy proteins break down and unify into a thick, cohesive, creamy spread. (Ensuring the cream cheese is fully softened is critical to prevent a lumpy foundation).

Pour the thoroughly drained diced pimentos directly into the cheese mixture. (Properly draining the pimentos prevents their packing liquid from breaking the fat emulsion and turning the dip watery).

Add the sliced green onions, garlic powder, and smoked paprika.

Using a rubber spatula, gently and deliberately fold the aromatics into the cheese base. You want the bright red pimentos and green onions to remain perfectly intact and suspended throughout the dip, providing critical textural contrast against the smooth dairy.

Taste and season aggressively with kosher salt and coarse black pepper.

Transfer the thick, highly structural spread into a serving bowl. Serve immediately at room temperature alongside toasted crostini and celery sticks, or chill in an airtight container for up to a week.