I picked this one up on America's Test Kitchen. I've made it twice now and it's amazing. A few notes... I haven't had sherry or apple butter on hand so I used white wine and just skipped the apple butter completely. If you want to make this vegetarian, leave out the marrow bones and swap the beef stock with veg stock. I've roasted the marrow bones in the oven, but when they made this on tv they put them in the microwave for 8 minutes. https://www.cookscountry.com/recipes/5893-slow-cooker-french-onion-soup