Preheat oven to 400 degrees
Heat a 7 quart or larger dutch oven on low (seriously, you need a pot this big, it makes a ton!) and melt butter.
Remove from heat and stir in onions and 1 tsp salt so that onions are coated.
Transfer Dutch Oven to lower center rack in oven, and cook covered for 1 hour.
Remove from oven and stir, scraping bottom and sides of pot.
Return to oven for 90 minutes with lid slightly ajar. Stir and scrape again half way through. (Onions should be very soft and golden brown.
Remove from oven and transfer to stovetop.
On medium heat, cook onions until they brown and a crust forms on the bottom of the pan (about 30 minutes) stirring frequently and scraping the pan as you go.
Stir in water, ¼ cup at a time scraping free crust, until it evaporates and crust reforms on pan. Repeat 3-4 times until onions are very dark brown.
Stir in wine and cook until evaporated (5-7 minutes)
Stir in chicken and beef broth, water, thyme, bay leaf and ½ tsp salt and bring to a simmer.
Reduce heat to low and simmer 30-40 minutes.
meanwhile, set oven to 425 and roast marrow bones 15 minutes, seasoned with salt and thyme.
while marrow is roasting, toast your slices of bread in the oven for 5-7 minutes
Remove marrow from oven and let cool.
Place 8 oven safe bowls on a baking sheet, fill with soup and top each with a slice of bread and raclette.
Switch oven to broil and with the top of the bowls 6-8" below broiler, broil until cheese is melted (approximately 5 minutes).
Serve with bone marrow on the side. The marrow can be spooned on top of the bread, or mixed into the soup per individual preference (I think it is delicious on top of the bread) follow link for the full marrow recipe.