In your gallon ziploc bag, combine the red wine vinegar, olive oil, monk fruit, garlic, dried thyme, salt and pepper. Shake and squeeze it to mix the marinade. Add the tri tip roast, remove as much air as possible, and seal. Marinate for at least 1 hour in the refrigerator, or up to 2 days.
Preheat your grill (or indoor grill grates) over medium high heat. Remove the tri tip from the marinade, discard any remaining marinade in bag. Sprinkle the tri tip with about 1 teaspoon of salt, and grill the tri tip covered for 15 minutes. Flip and continue to cook until the tri tip has an internal temperature of 135ºF for medium rare (or 145ºF for medium), about 8-10 more minutes depending on the thickness of your tri tip. Remove the cooked tri tip from the grill and rest for 10 minutes.
While the tri tip is cooking and resting, make the grilled onions. Heat a large skillet over medium heat. Add the olive oil, sliced onions, salt and pepper. Cook for about 30 minutes, stirring occasionally, until the onions are soft. If the pan ever gets too dry, or the onions start to get too charred, I add in 1/4 cup of water to keep everything juicy. The water will cook away. Add more water during the cooking process if needed. When the onions are softened, add in the 1 tablespoon of red wine vinegar and 1 teaspoon of thyme. Stir to combine. Remove from heat.
After the tri tip has rested, slice against the grain and serve topped with the grilled red onions.