Rinse herring under cold water, pat dry and sprinkle with lemon juice. Peel onions and cut into rings. Boil 400 ml (approximately 1 2/3 cups) of water with vinegar, 1 teaspoon salt, peppercorns, bay leaf, sugar and onion rings in a pan and cook until sugar has completely dissolved. Then pour in another 300 ml (approximately 1 1/4 cup) of water and allow sauce to cool.
Mix flour with 1/2 teaspoon salt and turn the herring in it to dust both sides. Heat oil in a large non-stick pan and fry herring on each side for about 4-5 minutes.
Place herring in an elongated bowl, pour over marinade, cover and refrigerate for 24 hours.
Serve with roasted potatoes, carrots and apple, as desired.