Sauté celery and shallots in 3 tbsp butter on medium heat until tender. Add thinly sliced spinach leaves; mix thoroughly until spinach is warm and cooked. Remove from heat, stir in cheese and bread crumbs. Let cool slightly then add in egg. Refrigerate until firm.
Preheat oven to 350°F Flatten breast halves with the side of a mallet until about 1/4 inch thick, Being careful not to tear through chicken. Place 1 heaping tablespoon of spinach mixture on center of each chicken piece, fold envelope style and secure with a plain wooden pick.
Place chicken seam side down in 9 x 12 baking dish. Sprinkle with salt and pepper. Heat butter, wine, tomatoes, chicken bouillon and thyme. Pour 1/2 of sauce over chicken.
Bake uncovered for 30 minutes. After 30 minutes, baste chicken with remaining sauce, then add sliced mushrooms into pan and spread around until they are coated with the sauce. Return to oven and bake until chicken is tender, about 20 minutes longer. Remove wooden picks from chicken, place chicken on serving platter and keep warm.
Pour drippings and remaining sauce from baking dish into saucepan. Blend cornstarch with cream and slowly stir into sauce until sauce thickens and comes to a low boil (add additional cream if needed to thin sauce). Season with salt and pepper.
Pour sauce over chicken trying to keep tomatoes and mushrooms on top of the chicken for aesthetic purposes. Serve with buttered noodles or rice pilaf.