First make the chilli butter so that it has time to firm up in the fridge. Mix the red chilli into the softened butter with the lemon juice and parsley. Transfer onto a sheet of non-stick baking parchment, shape into a roll about 4 cm thick and wrap up tightly. Chill for at least 1 hour until firm.
Preheat a cast iron ridged griddle pan or a barbecue. Place the steaks, red peppers and red onions in a shallow dish and sprinkle over the balsamic vinegar, olive oil and season with salt and pepper, turning to coat.
Arrange the vegetables on the hot griddle pan or barbecue. Cook the vegetables for 4-5 minutes, turning once. The steaks will take 2-3 minutes on each side for medium rare. The time will depend on the thickness of the steaks and how well done you like your meat. Leave to rest for a few minutes
Split the french stick and cut into four pieces or split the ciabatta rolls and fill with dressed salad leaves and the grilled vegetables. Top with the rested steaks and add a slice or two of the chilli butter. Arrange on plates to serve.