Combine flour, eggs, milk, melted butter, and salt in a blender. Blend until smooth, then strain through a fine sieve. Refrigerate for at least 30 minutes.
Heat a nonstick crepe pan over medium heat with a small amount of butter. Pour about 1/4 cup of batter into the pan, swirling to coat the bottom evenly. Cook for 1-2 minutes until golden, flip, and cook for another 30-45 seconds. Repeat with remaining batter.
Season chicken pieces with salt, pepper, and garlic powder. Heat olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Cook chicken in a single layer for 3-4 minutes per side until golden and cooked through. Transfer to a plate.
Add remaining butter to the skillet. Sauté onion for 3-4 minutes until translucent, then add garlic and cook for 30-60 seconds until fragrant. Add mushrooms and cook until tender and browned, about 4-5 minutes.
Pour in chicken broth and scrape up browned bits from the bottom. Add heavy cream and nutmeg, then simmer for 4-5 minutes until slightly thickened. Stir in Parmesan cheese until melted and smooth. Season to taste.
Return cooked chicken to the skillet and stir to coat. Add fresh spinach and cook for 1-2 minutes until wilted and fully incorporated. Remove from heat.
Preheat oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish. Place about 3-4 tablespoons of filling on each crepe, fold in the sides, and roll up tightly. Place seam-side down in the baking dish. Spoon any remaining sauce over the top and sprinkle with Gruyere cheese.
Cover with foil and bake for 15 minutes. Remove foil and bake for 5-8 minutes more until cheese is melted and bubbly. Garnish with fresh parsley and serve.