Preheat the grill to its highest setting.
For the chutney, heat the oil in a saucepan on a medium heat place the onion, red pepper and garlic into it and fry for five minutes, to soften.
Add the brown sugar to the saucepan and cook for a further ten minutes, stirring regularly.
Add the butter and vinegar to the saucepan and cover, allowing to cook gently for 5-10 minutes.
Meanwhile,for the cheese on toast, mix together the mustard and Greek yogurt in a bowl, then spread onto the slices of toasted ciabatta.
Sprinkle the cheese on top and place under the grill to cook for 2-3 minutes, or until the cheese has melted and is bubbling.
Remove the cheese on toast from the grill and place on a serving plate, with a spoonful of the chutney.