Heat saucepan over medium heat. When saucepan is warm, add butter and let butter melt completely. Swirl saucepan occasionally or move butter around with whisk to coat bottom of saucepan.
When butter is melted, but not bubbling or boiling, slowly add flour in small increments. Stir constantly to avoid lumps, and whisk well between each addition. Repeat until all flour is incorporated into melted butter.
Reduce heat to medium-low. Simmer roux 15 to 20 minutes or until roux is smooth, with rich brown color and slight nutty scent. Whisk frequently to avoid sticking or burning.
As soon as roux is ready, remove saucepan from heat. Let roux cool slightly, then slowly add half and half in small increments. Whisk well between each addition and repeat until all half and half is added and mixture is very smooth.
Return saucepan to heat. Increase heat to medium-high. Whisk constantly, scraping bottom of saucepan with whisk to reduce sticking, until mixture begins to boil.
Once liquid begins to boil, immediately reduce heat under saucepan to low. Continue whisking constantly until boiling subsides, then allow mixture to simmer 10 to 15 minutes. Whisk as needed.
After mixture simmers for desired length of time, remove saucepan from heat. Let mixture cool slightly, then add pieces of brie. Whisk until cheese is melted and incorporated, then return saucepan to low heat.
Heat large pot over medium-high. When pot is warm, add 8 ounces oyster liquor to pot.
Bring liquid to boil, then add shucked oysters. Boil oysters 3 minutes, or until ends of oysters begin to curl.
Use fork to remove cooked oysters from pot and transfer to cutting board. If oysters are large, halve or quarter oysters before continuing. Set cutting board aside.
Pour oyster liquor left in pot through sieve or colander and directly into saucepan of brie soup. Clean out pot if oysters stuck to bottom at all, then return pot to heat and reduce heat to medium-low.
Transfer brie soup from saucepan to pot. Add cooked oysters and ½ cup champagne to pot. Gently whisk soup to incorporate ingredients, then season with salt and pepper to taste.
Taste soup and add more champagne as desired, up to 1 cup champagne total. Gently whisk to incorporate.
When satisfied with flavor of soup, ladle soup into serving bowls. Garnish with fresh chives and serve warm with crusty bread.