Heat butter and oil over medium high heat in a large oven safe skillet (12 inches or more, see note). Add onions and cook undisturbed for 5 minutes, until onions start to sweat and have browned in places. Reduce heat to medium low, add salt and and cook for an additional 10-15 minutes, stirring occasionally, until the onions have softened. Add garlic and thyme and cook for an additional 2 minutes. Add in beef stock and mustard and scrape up any browned bits and continue to cook until the liquid has reduced by about half, about 2 minutes.
1 tablespoon butter, 1 tablespoon oil, ½ teaspoon fine sea salt, 5 medium onions, 3 garlic cloves, 3 sprigs fresh thyme, ½ cup beef stock, 1 tablespoon dijon mustard