In a small bowl combine ingredients for marinade: Marsala Wine, Olive Oil, garlic, Worcestershire Sauce, Tomato Paste, Rosemary and Salt and Pepper. Stir and mix to combine. Add 1 diced Onion and stir. Note* Reserve ¼+ cup of marinade too use when sautéing vegetables.
In a separate bowl, place Tri Tip Roast and pour marinade over the top, cover and place in the refrigerator for 4 - 24 hours. (The longer this can marinate the better)
Remove roast from refrigerator and allow to come to room temperature about 15-20 minutes before you want to grill.
Heat gas/charcoal grill to 400 degrees. Once the grill is hot, move all charcoal to one side, on a gas grill, turn off the heat on one side. Sear the roast on all sides under direct heat. This will take 3-5 minutes per side. Then move the roast to the other side (indirect heat) and allow to cook covered on the grill for an additional 15-25 minutes per pound.
Please note* Grilling times vary by grill and heat source. Use a digital thermometer to test doneness for desired preference.
Rare: 130-135 degrees
Medium Rare: 140 Degrees
Medium: 155 Degrees
Well Done: 165 Degrees
In a frying pan sauté' 1 sliced onion and mushrooms in reserved ¼+ cup marinade over medium high heat. Cook 4-6 minutes until desired doneness.
Allow roast to rest for 10 minutes. Keep in mind that the final cook temperature will increase while resting.
Slice Tri Tip Roast at an angle and serve over Sautéed mushrooms and onions.