Rinse fish thoroughly in cold water; pat dry. Rub fish with salt and pepper; set aside.
Spread onion, parsley, thyme, and bay leaf evenly in bottom of a lightly greased 13- x 9- x 2- inch baking dish. Place fish on top of vegetables; pour wine over top. Bake, uncovered, at 350° for 20 minutes.
Melt 2 tablespoons butter in a small saucepan over low heat; add flour, stirring until smooth. Cook over medium heat until lightly browned. Add tomatoes and mushrooms, mixing well; simmer 10 minutes.
Pour sauce over fish; cover with cracker crumbs. Dot with remaining butter. Bake at 350° an additional 10 minutes or until fish flakes easily when tested with a fork.
Garnish fish with lemon wedges: serve immediately.