To Make the Dukkah Spice Blend:

1. Place a large skillet over medium-low heat. Once hot, add hazelnuts and toast until fragrant, about 5 minutes. Pour hazelnuts onto a plate and allow to cool just sightly, about 2 minutes. 
2. Once cool enough to handle, rub handfuls of hazelnuts between your palms to remove the skins. Place hazelnuts in a food processor and set aside. 
3. Return skillet to the stove over medium-low heat and add sesame, coriander, cumin, and fennel seeds. Toast for 1-2 minutes, stirring often, until fragrant. Add seeds to food processor with hazelnuts. 
4. Add black pepper and salt to food processor. Pulse until mixture is coarsely ground. Set aside. 

To Make the Soup:

1. Preheat oven to 375°F and cover a large baking sheet with parchment paper. 
2. In a large mixing bowl, toss chopped carrots and onions with melted butter, salt, and pepper. Transfer them to the prepared baking sheet and spread out in a single layer. 
3. Place baking sheet in the preheated oven and roast until vegetables are tender and just starting to brown, about 45 minutes. Remove from oven and allow to cool on baking sheet for 10 minutes. 
4. Transfer the slightly cooled vegetables to a high-powered blender. Add vegetable broth and blend on high until smooth, about 2-3 minutes. 
5. Pour mixture into a stockpot and place over medium heat. Stir occasionally until hot, about 10 minutes. For a thinner soup, add water 1/4 cup at a time and stir between additions until it reaches your desired consistency. 
6. Once soup is heated through, divide evenly between four bowls. Top each with a sprinkle of the dukkah spice blend and a dollop of plain Greek yogurt.