Brush the salmon with lemon juice on both sides and cover completely with dill.
Wrap in plastic wrap and cool for about 1 hour at very low temperature until almost frozen.
Remove the salmon and cut diagonally into very thin slices, on a kitchen board,. Reserve neatly on a plate and keep very cold.
Toast the bell peppers directly over a flame or on a toaster until brown, and loosen and remove the skin.
(Once toasted, remove the skin by rubbing the pepper with your fingers using a rubber glove.)
Heat 2 tbsp of olive oil in a pan over high heat, brown the minced garlic clove for a minute, then add the peppers and cook for 3 minutes on both sides while they soften and brown. Add salt and pepper and finally the finely chopped parsley. Turn off heat and set aside.
In the meantime, cut the onion into thin rings, heat 2 tbsp of olive oil in a pan, and sauté the onion for 5 minutes over medium-high heat until golden brown. Add a few drops of vinegar and continue cooking for 1 minute more. Season and remove from heat.
Separate the ciabatta bread in half, spread both halves with strong mustard, cover with arugula, thin sliced salmon, roasted onions, and peperonata sauce.
Close with the other half of the ciabatta and serve immediately.