In a medium bowl, whisk together the all-purpose flour, spelt flour, cane sugar, salt, and fennel seeds. In a small saucepan over low heat, melt the butter. Pour melted butter into dry ingredients and stir to combine until mixture forms moist, clumpy crumbs. Place in freezer for 15 minutes to firm up. Adjust oven rack to middle position and preheat to 325°F. Line a baking sheet with parchment paper. Once oven is preheated, remove streusel from freezer and spread into a single layer. Bake for 20–25 minutes, shaking halfway through, until the edges begin to turn golden brown. The crumbs will be very soft straight from the oven, but they will firm up as they cool. If you like a crunchier crumb, you can increase the baking time to 25–30 minutes. Remove from oven and cool on wire rack. Store in airtight container at room temperature for up to a week, or freeze for up to 3 months.