Cut onions into fine rings or wedges. Heat butter and olive oil in a pan, add mustard seeds and sauté briefly. Add onions, bay leaf, chili flakes and herbs de Provence and slowly simmer over medium heat. This can take about 20-30 minutes. The onions should not be browned, just gently braised until soft. Season with salt and pepper. When the onions are tender, add the sugar and let it caramelize slightly.
Briefly increase the temperature lightly and douse the onions with balsamic vinegar and Taunus-Tropfen and allow the mixture to reduce slightly. Finally, season again with salt, pepper and if needed, a little more sugar. Pour hot into a clean jar.