Place tri tip into a flat shallow dish. Drizzle with oil on all sides.
In a small bowl, whisk together brown sugar, salt, pepper, red pepper flakes, granulated garlic onion powder, paprika and rosemary. Rub on all sides of tri tip. Place into a shallow dish and cover with plastic wrap. Chill overnight.
Roast in the Oven: Remove tri tip from fridge and rest on counter for 30 minutes. Preheat oven to 400°F.
Drizzle a 12 inch cast iron skillet or similar oven safe skillet with a couple drizzles of olive oil. Turn heat to high and heat pan for 1-2 minutes. Add tri tip to hot pan fat side down. Sear for 2-3 minutes then flip.
Add teriyaki sauce to pan, arrange mushrooms around tri tip and sprinkle with garlic. Place skillet into oven and roast for 25-30 minutes or until internal temperature of 130°F when inserted into thickest part of meat.
Remove from oven and tent with foil. Let stand 10-15 minutes, then slice across the grain and served drizzle with pan juices, topped with mushrooms.
Grill: Remove tri tip from fridge and rest on counter for 30 minutes. Preheat grill to high. Sear tri tip for 6-8 minutes on both sides starting with fat side down.
Lower temperature to medium-high. Flip tri tip and repeat grilling for 6-8 minutes on each side, grilling for 25-30 minutes total or until a meat thermometer registers 130°F.
Rest on cutting board for 10 minutes. Slice across the grain and serve.
Note: Mushrooms can be left whole and threaded onto skewers. Brush with oil and season with salt and pepper to taste. Grill along with tri tip until tender and charred.