Place vinegar, wine, shallots, raspberries, peppercorns, bay leaf, thyme, and lemon juice in a small, heavy saucepan. Bring to a boil over medium-high heat, then lower the heat and simmer until the contents is reduced by 90 percent. If using, whisk in the cream and continue cooking until the volume is further reduced by half. Take the cubed butter from the fridge and get ready for action.
Remove the saucepan from the heat and immediately start whisking in the butter, one piece at a time. Whisking continuously is imperative in achieving emulsion.
Strain the sauce through a conical chinois** and into a bowl. Embellish the serving plates with the beurre rouge as desired and serve immediately.
*A modest amount of dairy makes the sauce more stable without having a major impact on the taste.
**Investing in this type of fine-mesh metal sieve is a worthwhile acquisition for serious home cooks. Choosing one a with a stand makes it easier to use, and many include a suitably sized pestle for squeezing out liquid while leaving unwanted solids behind. Otherwise, using the back of a ladle is a perfectly fine hack.