Peel and halve the onions, then cut into thin rings. Clean the veal liver, remove any remaining skin, and cut into strips about 2 cm wide.
Heat the 3 tablespoons olive oil and 30 g butter in a large pan over medium heat for about 1 minute until the butter is foamy. Add the onions and cook, stirring frequently, for 5-7 minutes until softened.
Increase the heat to medium-high. Season the liver with salt and pepper. Add the liver to the pan and cook, turning frequently, for 2-3 minutes.
Pour the white wine into the pan and heat for 1-2 minutes until just below boiling (95-99°C / 203-210°F). Remove the pan from heat. Stir in the remaining 20 g butter until melted. Do not continue cooking. Taste and adjust seasoning if needed. Arrange on preheated plates and serve with polenta.