To make the salsa verde:
Place spinach, parsley, chives, egg and vinegar in a food processor and process until combined. With the motor running, gradually add the olive oil in a thin, steady stream and process until smooth.
Alternatively, combine all ingredients in a bowl, mash until smooth and rub through a sieve.
Cook potatoes in boiling water until tender.
Drain and cool slightly, then cut into 1cm slices.
Carefully fold the salsa verde through the warm potatoes.
Heat vegetable oil in a frying pan. Cook the fish in batches for 3-5 minutes each side, or until tender. Remove from pan and keep warm.
Reduce heat slightly, add butter and cook until just brown. Stir in garlic, peppers and lemon juice, and cook until soft.
Arrange fish with potatoes and peppers on a warm platter.