Preheat a Traeger grill or pellet smoker to 225ºF or an oven to 250ºF.
Remove the tri tip from the marinade and pat dry with a paper towel. Place directly on the grill grates or on a sheet pan with a wire rack in the oven. Pro tip: Insert a probe or wireless meat thermometer into the thickest part of the tri tip for constant internal temperature monitoring.
Cook until the tri tip reaches an internal temperature of 115ºF (for medium-rare) or 125ºF (for medium). On the Traeger, this will take 75-90 minutes. In the oven, it will take around 50-60 minutes. Note: If you're using a larger tri tip you've seasoned yourself, cook times will vary.
Once the tri tip has reached your desired temperature, heat a large skillet or griddle over high heat with a tablespoon of beef tallow or oil. Add the tri tip and sear all sides well, about 30-60 seconds per side.
Transfer the tri tip to a cutting board to rest and add the onion, peppers, and soy sauce. Cook for 4-5 minutes, stirring occasionally, until the veggies have softened and picked up a bit of color. Transfer the veggies to a large bowl and set aside.
After the tri tip has rested for 10-15 minutes, slice it thinly against the grain and cut the slices into 3-4 smaller strips. Add the sliced tri tip to the bowl with veggies, tossing everything together.
Optional: Add another tablespoon of beef tallow or oil to the skillet and sear the tri tip and veggie mix one final time before serving.