Slice the tri tip in the center to have two pieces, each has a different grain, so it will cook quicker, and slice more easily.
Slice the onion into thick slices (so they stay together when grilled.
In a gallon plastic bag, place the beef with onion, brown sugar, Worcestershire, lime juice, garlic, and salt. Marinate for 30 minutes to overnight. I've done it both ways. I prefer it overnight, but in a hurry, it's still nice and tasty.
Heat the grill until over 400 degrees° then place the meat on the grill and sear it on both sides, grill for a few more minutes. Add the sliced onion and zucchini or desired vegetable.
Turn the heat down to medium-high to medium heat. Cook until the meat is 135°F. Remove and allow the meat to rest with a piece of foil on top for 10 minutes, then slice and serve. It's delicious with mashed potatoes