In a blender, combine the all-purpose flour, eggs, whole milk, melted unsalted butter, and kosher salt. Blend until completely smooth, scraping down the sides as needed, about 1 minute.

Heat a 10-inch nonstick skillet over medium heat and spray with cooking spray. Add ¼ cup of the crêpe batter and tilt the pan to make sure the mixture evenly and lightly coats the skillet. Cook for 1 to 2 minutes, until the crêpe is lightly browned. Flip to cook the other side for about 1 minute. Set the crêpes aside to roll with filling or fold them into quarters and top with sauce. The crêpes can be made ahead up to 2 hours, covered, and refrigerated.

In a large skillet, heat 3 tablespoons of unsalted butter over medium-high heat. Add the sliced baby bella mushrooms and cook in one layer, without disturbing, until browned, about 4 minutes. Flip and continue cooking until tender and browned, stirring occasionally, about 5 minutes. Add the minced garlic and stir constantly until fragrant, about 1 minute. Stir in the heavy cream, garlic salt, and ground white pepper and simmer, stirring occasionally to check the consistency, until thickened, 5 to 8 minutes. Add the grated Parmesan cheese and stir until smooth and melted.

Measure out ¾ cup of the mushroom sauce and set aside in a small bowl. Add the baby spinach to the sauce remaining in the pan and cook, stirring often, until the spinach is wilted, approximately 2 minutes.

Fold in the shredded cooked chicken breast and cook until heated through, about 1 minute.

If filling the crêpes, spoon ¼ cup of the chicken mixture onto each crêpe and roll up.

Serve the filled or folded crêpes immediately, topped with the reserved mushroom sauce and chopped fresh dill.