Baked French Toast with Pecan Crumble

Name: Baked French Toast with Pecan Crumble
Ingredients: 1 pound loaf challah, sliced 1-inch thick; Unsalted butter, room temperature (for baking dish); 4 large eggs; 4 large egg yolks; 2 cups heavy cream; 2 cups whole milk; ¼ cup sugar; 1 teaspoon vanilla extract; ½ teaspoon ground cinnamon; ½ teaspoon ground nutmeg; ½ teaspoon kosher salt; ½ cup pecans; 2 tablespoons chilled unsalted butter, cut into pieces; 2 tablespoons light brown sugar; ¼ teaspoon kosher salt; Blackberry Syrup (for serving; click for recipe)