For pickled Spanish onion, combine 1 tbsp red wine vinegar and sugar in a small saucepan. Stir over medium heat until sugar dissolves, remove from heat, add remaining vinegar, onion and coriander seeds. Set aside until lightly pickled (15 minutes).
Preheat a char-grill over medium-high heat. Spread one side of bread slices with butter and turn over. Spread half the slices (unbuttered side) with goat’s cheese and sprinkle with grated cheeses. Sandwich with remaining bread (buttered side up). Grill sandwiches, pressing with a spatula until golden and cheese melts (1-2 minutes each side). Serve with pickled Spanish onion.