Place the fennel in a bowl along with the peas, broad beans, radishes and watercress.
Make the dressing by mixing together the mustard, honey, vinegar and oil. Season to taste with salt and pepper. Add enough of the dressing to the salad to coat all the ingredients. Set aside while you cook the plaice.
Heat a little of the oil in a very hot frying pan and dust the plaice fillets with the seasoned flour. Place the fish, skin-side down, in the pan. Cook for 3 minutes on the skin side, then carefully turn the fish over and cook for a further 2 minutes. (Depending on the size of your pan, you may need to cook the fish in 2 batches.) Once the fish is cooked, squeeze a little lemon juice over the fish and serve with the fennel salad.