Sprinkle 1/4 cup Cheddar cheese in pastry shell. Mix chicken, spinach, onion, and remaining 3/4 cup Cheddar cheese in a bowl; transfer mixture to pastry shell.
Whisk eggs, half-and-half, mayonnaise, salt, black pepper, seasoning blend, and herbes de Provence in another bowl; pour over chicken mixture.
Bake in the preheated oven until a knife inserted into the center comes out clean, 35 to 40 minutes. Let stand for 15 minutes before slicing and serving.
