Open the herring skin-side up and flatten out the fish. Turn over, then spread the flesh side with mustard and press in the oats.
Place under a hot grill, oat-side down for 4-5 minutes, then turn the fish over and grill for another 2-3 minutes to brown the oats
Mean while, peel and slice the onion into thin rings and add to a pan with half the oil, cook for 5 minutes until softened
In a separate pan, with the spinach with the remaining oil, drain and arrange on plates, sprinkle onions on top and squeeze over the lemon juice
Place the herring oat-side up on the spinach and sprinkle with capers.