For the fillet of beef, preheat oven to 110 °C. Halve the shallots, lightly press the garlic clove. Sear the beef fillet on all sides in the hot clarified butter, season with salt and pepper. Add thyme, rosemary, shallots and garlic and fry briefly. Put the ingredients on a tray lined with aluminum foil and cook in the oven on the middle rack for 30-50 minutes.
For the napkin dumplings, dice the shallots very finely. Saute in a pan and transfer to a bowl. Coarsely dice the pretzel bread and add to the shallots. Whisk the egg with the milk. Season with the spices and add to the pretzel bread, stir and let stand.
Put the dumpling mixture on a plastic wrap, form a sausage and wrap with aluminum foil. Cook in boiling water for about 25 minutes.
For the onion sauce, cut onions in half into fine rings and cut in half again. Fry in olive oil, remove rosemary needles, chop finely. Press the garlic and add to the onions with the honey, stirring occasionally.
Dust the onions with flour, sauté briefly and deglaze with red wine. Reduce, add the balsamic vinegar and stir. Add the beef broth and stir to thicken to a creamy consistency. Season to taste with salt and pepper.
Cut potatoes into thick slices using a heart cutter to cut out hearts. Boil the potatoes and serve with a carrot dart with the beef tenderloin and napkin dumplings.