1. Melt butter in a 12-inch nonstick skillet over medium heat. Add onion and sauté until softened, about 4 minutes. Add tomatoes and chiles and cook, stirring occasionally, for 10 minutes. Reduce heat to low and add cheese in 4 batches, stirring constantly, until melted. Stir in salt.
2. Transfer fondue to a fondue pot or chafing dish, adjust heat to keep fondue hot, sprinkle with scallions and serve hot with tortilla chips and crudités, if desired.