Melt the butter in a pan over medium heat. Add the grapes and cook for 2-3 minutes, stirring occasionally, until heated through. Transfer to a warm plate.
Cut the chicken liver into bite-sized pieces and coat lightly with flour. Season with salt and pepper.
In the same pan, fry the liver pieces over medium-high heat for 3-4 minutes until cooked through (internal temperature should reach 74 °C / 165 °F). Transfer to the plate with the grapes and keep warm.
Add the finely diced shallots to the pan over medium heat and cook for 2-3 minutes, stirring occasionally, until translucent.
Deglaze the pan with the Madeira or port wine and simmer for 2-3 minutes until slightly reduced.
Return the grapes and liver to the pan, toss gently to combine, heat through for 1 minute, and serve immediately with baguette and a well-chilled Riesling or Pinot Gris.