Preheat grill to medium-high heat.
While the grill comes to temperature, heat olive oil in large pan over medium heat.
Add in chopped celery, onions, and carrots. Cook 3-4 minutes or until onions are translucent. Add garlic, red pepper flakes, salt and pepper and stir for another 30 seconds. Remove from pan and place aside in a bowl.
Add ground beef to pan and cook 5-6 minutes or until no longer pink. Add in bone broth and taco seasoning. Continue to cook for another 2-3 minutes or until most of the liquid is gone. Add veggies back in along with the fresh thyme and cumin. Stir until well incorporated.
Spoon equal amounts of ground beef mixture into each half of a red bell pepper.
Place stuffed bell peppers on the grill for about 10-12 minutes or until peppers are slightly soft and grill marks have formed.
Remove peppers from the grill. Garnish with chopped romaine lettuce and chopped avocado.