Preheat the oven to 425 degrees F (220 degrees C).
Form sausage into mini meatballs and place on a rimmed baking sheet.
Bake in the preheated oven until nicely browned, 15 to 18 minutes. Transfer sausage to a paper towel-lined plate.
Reduce oven temperature to 350 degrees F (175 degrees C). Grease a deep-dish pie pan or a 2-quart casserole dish.
Combine boiling water and soup base in a bowl to create a double-strength chicken stock. Set aside.
Melt butter in a skillet over medium heat; stir in onion. Cook and stir until the onion has softened and turned translucent, about 5 minutes. Add mushrooms and bell pepper. Cook, stirring occasionally, until mushrooms begin to soften, 5 to 8 minutes. Add garlic; cook and stir until fragrant, about 1 minute.
Sprinkle 1/4 cup flour over the vegetables and stir until all are coated. Cook and stir for 2 minutes. Add reserved chicken stock and cream. Cook and stir until thickened, about 5 minutes. Stir in sausage, spinach, and cooked chicken. Pour mixture into the prepared pan.
Combine remaining flour, celery seed, paprika, and salt in a medium mixing bowl. Cut in shortening with a pastry knife or fork until mixture looks like coarse meal. Sprinkle ice water over flour mixture and stir unto a ball.
Transfer dough onto a lightly floured surface. Flour a rolling pin and roll dough into a circle large enough to cover the prepared pie pan or casserole dish. Carefully place dough on top of chicken filling and make several slits in the top.
Bake in the center of the preheated oven until filling is bubbly and crust is golden brown, 40 to 45 minutes. Cook on a rack for 10 minutes before serving.