Prepare Spinach mushroom Stuffing; set aside.
In cup, mix thyme, salt, and pepper; rub mixture all over tenderloin.
Turn thinner end under tenderloin to make meat an even thickness.
With sharp knife, cut 1 1/2 inch deep slit in tenderloin, starting 2 inches from thicker end and ending 2 inches from thinner end.
Preheat oven to 425°F.
Spoon Spinach mushroom Stuffing into slit in tenderloin.
With string, tie tenderloin at 2 inch intervals to help hold its shape.
Place stuffed tenderloin on rack in large roasting pan (17" by 11 1/2"); roast tenderloin 30 minutes.
Meanwhile, in small saucepan, melt 1 tablespoon margarine or butter over low heat.
Remove saucepan from heat; stir in bread crumbs.
Set bread crumb topping aside.
Remove tenderloin from oven; sprinkle breadcrumb topping over stuffing.
Roast tenderloin 10 to 15 minutes longer, until bread crumb topping is golden and meat thermometer reaches 140°F.
Internal temperature of meat will rise to 145°F. (medium rare) upon standing.
Or, roast to desired doneness.
Transfer tenderloin to large platter; let stand 10 minutes for easier slicing.
While tenderloin is standing, prepare mushroom gravy: Add vermouth and 1/2 cup chicken broth to drippings in roasting pan and stir over low heat until browned bits are loosened from pan.
Pour drippings mixture into 4 cup measuring cup; let stand a few seconds until fat separates from meat juice.
Skim fat from meat juice mixture; add remaining chicken broth and enough water to equal 2 1/2 cups; set aside.
In 12 inch skillet, heat remaining 1 tablespoon margarine or butter over medium high heat.
Add mushrooms and cook until golden and liquid evaporates, about 12 minutes.
Stir in flour.
Gradually stir meat juice mixture into mushrooms and cook, stirring constantly, until gravy boils and thickens slightly; boil 1 minute.