Preheat the broiler and position the oven rack about 4–6 inches below the heat source.
Arrange the baguette slices in a single layer on a baking sheet.
Broil the bread slices just until lightly toasted, about 1 minute—watch closely to avoid burning.
While the bread toasts, in a medium bowl combine the marinated artichoke hearts (drained and chopped), diced tomato (seeds removed), grated Romano or Parmesan cheese, finely chopped red onion, crushed garlic, and mayonnaise; mix until evenly combined.
Remove the toasted bread from the oven and top each slice with an equal amount of the artichoke mixture, spreading it to cover the entire surface (including the edges) to help prevent burning.
Return the topped slices to the broiler and broil about 2 minutes, or until the topping is bubbly and lightly browned—watch closely.
Remove from the oven and let cool briefly before serving.