Combine soy, honey, sugar, vinegar, ginger, garlic and peppercorns.
Pour over liver and let sit and for at least 2 hours (but preferably overnight), stirring occasionally.
Remove liver from marinade, shaking off the peppercorns.
Strain marinade and dispose of solids.
Combine strained marinade with cornstarch and stock and then set aside.
Heat oil and saute onions and mushrooms until limp.
Add onions and mushrooms to marinade.
Saute liver until lightly coloured.
Pour marinade-onion mixture into pan and cook until thickened, stirring constantly.
Serve.
