4 fish fillets, lowfat (such as perch or cod)
1 onion, finely chopped
1 Tbsp lemon juice
1 pinch salt and pepper
2 tsp Dijon mustard
1½ tsp corn starch
1 Tbsp olive oil or canola oil
1 cup evaporated skim milk, canned
2½ oz fresh button mushrooms, sliced
3 shallots, finely chopped
