Peel and cut all the onions root to stem to get nice 1/4-1/2 inch thick slices
Heat the butter in a large pot over very low heat (2/10) and when melted add the onions and a big pinch of salt. Turn and toss to coat the onions
For the next 2 hours toss the onions and scrape the bottom of the pot every 15 minutes. If you notice that any of them are taking on color lower the heat, and/or add a splash of your broth to slow them down.
When the onions have caramelized to your liking. Add the flour and stir well to combine. Let this cook for 3-4 minutes as is.
Add the beef broth, beef paste, chicken broth, worcestershire sauce, bay leaves and thyme to the pot slowly. Stir to combine and bring to a light simmer.
Stir the pot a few times over the next 30 minutes and get ready to serve. The soup is technically done as soon as you add all the ingredients but a solid 30-60 minutes here really makes the flavor come out. You can leave this on very low heat simmering for however long you want. Or, turn the heat off after 30 minutes and then back on to warm through when ready to serve.
While the soup is simmering, slice a baguette into 1/2 inch thick slices, brush or rub some olive oil on both sides and bake in a preheated 350deg oven for 5-6 minutes until starting to get crispy.
When the bread is toasty, add the sliced cheese and either put back in the 350deg oven until melted, or turn on the broiler and melt the cheese. Optionally you can torch the cheese if you have a kitchen torch.
To serve, ladle in the soup and top with however many slices of the cheese toast you can fit on top of the bowl. Keep an eye out for the bay leaves and thyme sprigs.