Preheat your oven: Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or non-stick spray to prevent sticking.
Prepare the bread: Slice the French baguette into 1-inch thick pieces and layer them evenly in the prepared baking dish, filling the base and ensuring there are no large gaps.
Make the egg mixture: In a large mixing bowl, whisk together 6 large eggs, 1/2 cup milk, 3 Tbsp brown sugar, and 1/2 tsp cinnamon until fully combined and frothy.
Pour over bread: Pour the egg mixture evenly over the layered bread, ensuring each piece is soaked nicely. Press down gently if necessary to allow the bread to absorb the egg mixture.
Create the topping: In a medium saucepan over low heat, melt 1/2 cup butter. Once melted, mix in 1 cup brown sugar and 1/2 cup corn syrup. Stir until fully combined and smooth, then add 1 tsp cinnamon and 2 cups pecan halves, stirring to coat the pecans well.
Assemble the casserole: Pour the pecan mixture evenly over the soaked bread in the baking dish, spreading the pecans as necessary to cover all the bread.
Bake the casserole: Place the baking dish in the preheated oven and bake for 40-43 minutes, or until the top is golden brown and the center is set. A toothpick inserted in the center should come out clean.
Serve warm: Remove from the oven and let it cool for a few minutes before serving. Enjoy your Pecan Pie French Toast Casserole warm, perhaps with a drizzle of maple syrup if desired!