Finely chop a large shallot and place it in a sauté pan with white wine, bay leaf, and thyme. Reduce gently by half, then add veal jus and heavy cream. Simmer until the sauce coats the back of a spoon. Remove from heat, stir in strong mustard (4 tbsp), strain, and add Meaux mustard (2 tbsp). Season to taste and set aside.
Cut sweet red onion into rings, blanch briefly, and fry in sunflower oil until golden. Set aside.
Trim fat from the veal kidney, halve lengthwise to remove white parts, and slice crosswise into thick strips. Season with salt and pepper. Sear in a hot pan with oil for 2 minutes per side. Drain on a colander.
Warm the lentil purée and shape into quenelles. Arrange kidney slices in a circle beneath. Reheat sauce, add mustard seeds, and mount with butter. Spoon over kidney, surround with sauce cordon, and garnish with fried onions, diced tomatoes, and chopped chives.
For the lentil mash: Sort and rinse lentils. Cover generously with cold water, bring to a boil, and skim. Season with salt and pepper, add bouquet garni, onion studded with clove, diced carrot, and mahaleb seeds. Simmer covered until tender. Discard aromatics, pass through a vegetable mill while hot. Reheat purée, stirring with a wooden spoon, and enrich with butter.
