Stuffed onion with Grana Padano and black truffle

Name: Stuffed onion with Grana Padano and black truffle
Cuisine: European, Mediterranean
Ingredients: 4 medium/large onions; 4 egg yolks; 40 gr of butter; 1 bay leaf; 15 gr of rice flour; 20 gr of black truffle (optional); 100 ml of white wine; Salt and pepper to taste; Rock salt to bake the onions; For the Grana Padano béchamel; 500ml of milk (warm, if possible) can use oat milk; 40g of butter; 40 g of rice flour; 200g of grated Grana Padano Riserva; Salt and pepper to taste; 1.5 tsp beef bullion paste