Mix 1/4 cup (60 mL) cold water and the cornstarch in cup until smooth; reserve.
Trim and slice green onions.
Mince garlic.
Reserve.
Scrub oysters thoroughly with stiff brush under cold running water.
Shuck oysters over bowl to catch liquor; reserve shells for garnish, if desired.
Strain oyster liquor through triple thickness of dampened cheesecloth; reserve.
Rinse scallops under cold running water; drain and pat dry.
Cut scallops into 1-inch (2.5 cm) pieces.
Shell shrimp under cold running water.
Cut a shallow slit down the back of shrimp; pull out and discard intestinal vein.
Pat shrimp dry.
Heat butter in large noncor-rosive skillet over medium heat.
When foam subsides, add oysters; saute just until edges begin to curl, 1 to 2 minutes.
Remove oysters with slotted spoon to plate.
Add scallops and shrimp to pan; saute just until opaque, 2 to 3 minutes.
Remove scallops and shrimp with slotted spoon to separate plate.
Reserve drippings in pan.
Cook spaghetti in large kettle of boiling salted water just until al dente, 8 to 12 minutes; drain well.
While spaghetti is cooking, saute reserved garlic in drippings remaining in skillet over medium heat 30 seconds.
Stir in wine; then stir in reserved oyster liquor and the cream.
Heat to boiling.
Cook over medium heat, stirring and scraping brown bits from bottom of pan, 5 minutes.
Stir in reserved cornstarch mixture; simmer over low heat, stirring constantly, 3 minutes.
Stir in lemon juice, salt and pepper.
Stir in shrimp and scallops; cook just until heated through.
Pour sauce over spaghetti in heated serving bowl.
Top with reserved green onions, the parsley and oysters; toss.
Garnish with reserved shells, if desired.