Heat a splash of oil in a large pot over medium-high heat. Add the diced onion and sauté for 3-5 minutes until translucent. Stir occasionally to prevent sticking. Then add the chopped garlic and thyme, cooking for another minute until fragrant.

Pour in the broth and apple cider, then add the diced apples. Bring to a boil while stirring, then reduce heat to simmer for about 20 minutes until apples are tender.

Stir in vanilla extract, maple syrup, shredded cheddar, and sliced brie. Keep over low heat for about 5 minutes, stirring often, until cheeses are melted and create a creamy texture.

Carefully transfer the mixture to a blender and blend until smooth, about 1-2 minutes. Optionally strain through a fine mesh sieve for a silkier texture. Season with salt and pepper to taste.

In a separate bowl, pulse the melted butter, flour, brown sugar, salt, and cinnamon together until grainy. Fold in rolled oats and chopped walnuts by hand.

Spread the granola mixture on a baking sheet and bake at 375°F (190°C) for 10-20 minutes, stirring every 5-10 minutes until golden and crisp.

Ladle the soup into warm bowls and sprinkle with the baked granola topping. Serve with crusty bread or a light salad.