Step 1: Caramelize the onions. In a large Dutch oven or heavy-bottomed pot, melt 2 tbsp butter and 1 tbsp olive oil over medium-low heat. Add the sliced onions and cook slowly for 30-45 minutes, stirring occasionally, until they are deep golden brown and wonderfully sweet. Be patient, don't rush this step!
Step 2: Sauté the mushrooms. Increase heat to medium. Add the sliced mushrooms to the pot and cook until they release their liquid and then brown, about 8-10 minutes. If your pot is too crowded, cook mushrooms in two batches.
Step 3: Add aromatics. Stir in the minced garlic and fresh thyme. Cook for another 1-2 minutes until fragrant.
Step 4: Create a roux. Sprinkle the flour over the onion and mushroom mixture. Stir well and cook for 2 minutes, allowing the flour to absorb the fats and cook out any raw taste.
Step 5: Build the soup base. Gradually whisk in the vegetable broth, ensuring there are no lumps. Bring the soup to a gentle simmer, then reduce heat to low, cover, and let it simmer for at least 15-20 minutes to allow the flavors to meld.
Step 6: Finish the soup. Remove from heat. If desired, stir in the heavy cream for extra richness. Season generously with salt and freshly ground black pepper to taste.
Step 7: Prepare cheesy toasties. While the soup simmers, butter one side of each bread slice. On the unbuttered side, generously top with grated cheese. Heat a large skillet or grill pan over medium heat. Place the bread butter-side down and grill until golden brown and the cheese is melted and bubbly, about 3-4 minutes per side (start with buttered side down, then flip to melt cheese).
Step 8: Serve. Ladle the hot soup into bowls and serve immediately with the cheesy toasties on the side or floating on top. Enjoy your ultimate comfort meal!