Saute mushrooms in 1 Tablespoon butter in a skillet over medium high heat. Set aside.
In a saucepan over medium heat melt butter, whisk in flour until incorporated and has formed a roux. Slowly whisk in the milk. Do NOT boil as this may scorch the sauce base.
Whisk in the dijon mustard, cream cheese, dried dill, and granulated garlic. Whisk in the shredded parmesan cheese a little at a time.
Pour 1/3 of the cheese sauce into the bottom of a greased 9"x13" casserole dish. Top with the tilapia fillets. Spoon the sauteed mushrooms on top of the tilapia. Slowly pour the remaining cheese sauce on top of the tilapia fillets. Bake for about 20-25 minutes.