Cut the baguette into equal size chunks and put it in a bowl. Add the olive oil, salt, and pepper. Mix with your hands and set aside.
Add the olive oil, onion, and garlic in a large skillet. Leave for three minutes and add the leeks and celery.
Leave until all the vegetables are completely soft, and they are beginning to stick a little bit at the bottom of the pan.
Add the wine, loosen the pieces of vegetables that stuck to the pan, and let it evaporate. Add the kale and salt.
Leave until the kale is incorporated, but do not let them completely wilt.
Put this mixture of vegetables on a baking dish and set aside.
Clean the pan you are using with a paper towel.
Put the olive oil or butter, and as soon as it melts, add the flour. Do not stop whisking, and when it begins to smell nutty and delicious. Be careful not to burn it.
Add the milk. Do not stop whisking, it will thicken quickly. Add the mustard, pepper, salt, and herbs. Test and adjust if necessary seasoning. Pour over the vegetables and mix well.
Cover with the pieces of bread and bake for 15 minutes or until the bread is golden brown.