Remove the seasoned Tri Tip from refrigerator 1 hour before grilling to bring to room temp.
Set burners to high and preheat grill to 500 Degrees Fahrenheit. Place Tri Tip on grill and sear 4 minutes on each side. Turn grill down to medium heat, close lid and cook until internal temp reaches 130-140 degrees Fahrenheit. Remove from grill and let rest for 10 minutes.
Cut and sauté your onions and red peppers
Slice pretzel rolls and Tri Tip against the grain into slices. Spread BBQ Sauce on grilled bread, add Tri Tip, onion, red peppers and Spinach.