Preheat the oven to 350F.
Make the waffle batter: In a large bowl, sift flour, baking powder, 1 tablespoon sugar and salt to combine. In a small bowl, vigorously whisk the egg yolks until smooth then add the brown sugar, milk, oil, lemon extract and lemon zest. Whisk until brown sugar is dissolved.
Make a well in the center of the dry ingredients. Pour in the liquid ingredients and gently stir the batter with a wooden spoon, until just combined. (You can use a hand beater or a stand mixer on low speed, as well.) Be careful not to overmix the batter. In a very clean bowl, whisk the egg whites until foamy. Add the remaining tablespoon of sugar and beat until soft peaks form. Gently fold the whites into the batter.
Cook the waffles: Heat a waffle iron and coat both the top and bottom surface with cooking spray. Pour about 1/4 -1/2 cup batter into the waffle iron and close. Cook until crisp and golden brown, about 5-6 minutes, depending on waffle iron settings. Allow to cool, then toast waffles in a toaster to remove as much moisture from them as possible. Or, put them directly on the oven rack and toast them at 150F until a tap feels more hard than soft. Once toasted, use the squares as a guide and cut each waffle into its individual squares.
Mix the custard: In a large bowl, whisk together the heavy cream, maple syrup, buttermilk, scallions, Hungarian paprika, thyme, garlic and a few grinds of coarse black pepper. Taste and season with a pinch or two of salt, then add eggs and whisk to combine. Add chicken, waffle squares and stir. Allow to sit for 10 minutes while stirring here and there to allow the custard to soak into the waffles.
Assemble the casserole: Butter an 8x8 baking dish. Pour the chicken and waffle mixture into the dish and pat down gently.
Top and bake the casserole: In a medium bowl, add the maple syrup, lemon zest and juice. Whisk to combine and add French-fried onions. Fold to coat then sprinkle topping evenly on top of the casserole. Bake until crust is golden and crispy in parts and custard is set, about 25-30 minutes. Allow to cool 10 minutes before scooping and serving with a drizzle of maple syrup.