Discard water from previous ice bath and prepare fresh ice bath. Set aside. Line large bowl with paper towels. Set aside,
Fill saucepan with enough water to cover onions completely and season water with plenty of salt. Place saucepan on stovetop over high heat and bring salted water to boil.
When water begins to boil, immediately reduce heat to medium-low. Simmer, uncovered, 10 minutes or until onions are tender.
Once onions are tender, drain water and transfer cooked onions to fresh ice bath. Use hand or spoon to swirl onions in ice water until onions are cool enough to handle.
Use slotted spoon to remove pearl onions from ice bath, allowing any excess water to drip back into bowl, then transfer onions to bowl lined with paper towels. Set aside.
Heat saucepan again, this time over medium heat. When saucepan is hot, add 3 tablespoons butter and melt completely. Be careful not to burn butter.
Add 3 tablespoons all-purpose flour to melted butter. Whisk until flour is fully incorporated into butter, creating roux. Cook roux approximately 5 minutes, whisking frequently to prevent burning. Season roux with salt and freshly cracked black pepper as desired.
Remove saucepan from heat and let cool slightly, then pour in 1 ½ cups light cream. Whisk to incorporate, then return saucepan to heat and cook 5 minutes more, until mixture is bubbly and thick. Stir as needed to prevent burning.
Add 2 tablespoons dry sherry (optional), 2 tablespoons grated fresh parmesan, 2 tablespoons chopped fresh parsley, and freshly cracked black pepper to taste. Stir until ingredients are combined and thick cream sauce forms. Note: If sauce is too thin, add more parmesan and cook until thickened. If sauce is too thick, add more cream or sherry until desired consistency is achieved.
Return cooked onions to saucepan and gently fold onions into sauce. Cook, stirring occasionally, 2 to 5 minutes or until onions are heated through.
Transfer prepared creamed onions to serving dish. Garnish with additional chopped fresh parsley and additional grated fresh parmesan if desired and serve immediately.