Combine the onions, garlic, white wine, and lemon juice in a non-reactive saucepan over high heat and reduce to 2 tablespoons. Add the cream to the reduction. Once the liquid bubbles, reduce the heat to low. Add the butter, one cube at a time, whisking first on the heat and then off the heat. Continue whisking butter into the reduction until the mixture is fully emulsified and has reached a rich sauce consistency. Season with salt and white pepper. Serve.