Thinly slice 5 sweet onions. Heat a thick-bottomed pot, or Dutch oven, on medium. Add 2 tbsp butter and 1 tbsp olive oil. When the butter foams, add onions and season with 1 tsp salt. Increase heat to medium-high and let the onions cook, stirring occasionally, until they've reduced in volume by about half. Fill a measuring cup with 1 cup water, and place next to the pot. Reduce heat to medium, and continue cooking like this, adding a tbsp or two of water as needed, until they are deeply caramelized, about 35 minutes.
Preheat oven to 400°F and line a baking sheet with parchment. Meanwhile, tear 600g oyster mushrooms into bite-sized pieces and dice 4 cloves garlic. Transfer the mushrooms to the baking sheet and cover with a generous pinch of salt and 2 tbsp olive oil. Toss to coat, then bake until golden and crispy, about 20 minutes.
Back in the pot, the onions should be about half-way finished. Add the garlic and continue cooking until deeply caramelized. Deglaze with 2 tbsp sherry vinegar and cook until the vinegar mostly evaporates and the aroma is more sweet than vinegar-y. Add in 5 sprigs of fresh thyme and 2 bay leaves. Cover with 4 cups beef broth and 1 cup water. Add 1 tbsp miso to a small fine mesh sieve. Using a spoon mix the miso into the broth, pushing it through the sieve first to prevent clumping. Bring to a simmer and cook for 20 minutes.
Meanwhile, grate 100g Gruyère and cut 8 slices from 1 fresh baguette. Remove the mushrooms from the oven and set aside to cool. Place a rack in the upper third of oven and turn broil to high.
Transfer soup to oven-safe ramekins, top with 2 baguette slices, a handful of roasted mushrooms and enough grated Gruyère to generously cover the top. Broil until completely melted and starting to brown all over, about 5 minutes.
Serve hot with more fresh thyme sprinkled on top.
