Arrange onions in an oven dish with their skins on, sprinkle with sea salt and bake at about 200°C. Peel, mince, add other ingredients and add salt and pepper to taste.
Peel onions, cut in half and soak in cold milk for two hours. Drain and put in a vacuum with other ingredients. Cook for about 1 ½ h bain-marie (85°C). Purée in the blender and add salt and pepper to taste.
Peel shallots and cut lengthwise into 6. Cook all ingredients except Xanthan gum until reduced by about half, add salt and pepper to taste, thicken with Xanthan gum and gently add shallots then continue cooking.
Clean veal sweetbreads and mince, sauté in butter until crunchy and add spices. When finished cooking, add rosemary, thyme, and a little garlic and then stir with veal broth and marjoram and serve.