Preheat oven to 325°F (160°C).
Make small slits in the roast and insert the garlic cloves.
Season the roast generously with salt and pepper.
Sear the roast in a hot skillet for 2 to 3 minutes per side.
Mix the Dijon mustard and margarine, then brush the mixture all over the roast.
Steep the tea bag in 1 cup hot water for 5 minutes.
Combine the brewed tea, beef broth, red wine, onion salt, celery salt, salt, and pepper.
Place the roast in a baking dish.
Scatter the sliced raw onions directly over the roast.
Pour the sauce mixture around the roast.
Cover loosely and roast for 1½ to 2 hours, basting 2 to 3 times during cooking.
Uncover during the last 10 minutes to lightly brown the onions.
Remove the roast from the dish and set aside.
Pour the cooking juices into a saucepan and heat over medium heat.
Add the gravy browning sauce diluted in a little water and stir until thickened.
Slice the roast and serve with the gravy and onions.