Grease a 9x13-inch baking dish with butter or non-stick spray. If your bread is fresh, cube it into 1-inch pieces and dry in a 300°F (150°C) oven for 10 minutes or leave uncovered on the counter for 1–2 hours, then spread evenly in the prepared dish.
Peel, core, and slice the pears into ½-inch pieces. Scatter them evenly over the bread cubes, gently tucking some slices down between the pieces so pear is distributed throughout the dish.
In a large bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, maple syrup, vanilla extract, almond extract (if using), cinnamon, nutmeg, and salt until completely smooth and uniform, about 60–90 seconds.
Pour the custard evenly over the bread and pears, then gently press the bread cubes down with a spatula so every piece is saturated. Cover tightly with plastic wrap and refrigerate for at least 4 hours, or overnight (8–12 hours) for best results.
In a medium bowl, combine flour, brown sugar, cinnamon, and salt, then cut in the cold butter cubes using a pastry cutter or fork until the mixture resembles coarse, pea-sized crumbs. Fold in the sliced almonds and refrigerate until ready to use.
Remove the casserole from the refrigerator and let it sit at room temperature for 20–30 minutes. Preheat your oven to 350°F (175°C) with a rack in the center position.
Remove the plastic wrap and sprinkle the almond streusel evenly over the entire surface of the casserole, then scatter turbinado sugar on top if using. Bake uncovered for 45–55 minutes until deep golden brown on top and the center is set (not jiggly); tent loosely with foil after 35 minutes if the topping browns too quickly.
Remove from the oven and let the casserole rest on a wire rack for 10 minutes before slicing. Dust with powdered sugar if desired and serve warm with maple syrup, whipped cream, or crème fraîche on the side.