Heat the oven to 350 F. Lightly butter a 2-quart baking dish.
Drain the cooked spinach in a colander, squeezing or pressing to get as much of the water out as possible. Spread the spinach in the bottom of the prepared baking dish.
In a medium saucepan over medium-low heat, melt 3 tablespoons of butter. Add the flour and seasoned salt to the butter and cook, stirring, for 2 minutes. Gradually add the milk and cook, stirring constantly, until thickened. Stir in the Parmesan cheese and cream. Cook, stirring until the cheese has melted. Taste and add salt and pepper, as needed. Add the chicken to the sauce and stir to blend.
Pour the chicken and sauce mixture over the spinach layer in the baking dish.
Melt the remaining 2 tablespoons of butter in the microwave or in a saucepan on the stovetop. Toss the butter with the soft breadcrumbs. Spread the crumbs over the casserole.
Bake the casserole for 20 to 25 minutes, or until the topping is nicely browned. To brown the topping a little more, turn the broiler on and broil for a minute or two before you take it out of the oven.