Heat grill to medium-high and brush or spray with oil.
Put peppers, cut side down on the grill and cook until cut side is just starting to char 4-5 minutes
Remove peppers from the grill and place in a cast-iron or other grill proof-skillet, cut side up and set aside.
In a skillet, heat olive oil on medium and cook onion and garlic until softened.
Add paprika and cayenne, and cook, stirring until fragrant.
Add ground beef and cook, breaking up with a spoon until no longer pink, about 5 minutes. Remove from heat and stir in farro, tomato, salt and pepper.
Spoon filling into the cut peppers, pushing with a spoon to fill each pepper, with a mound on top. Return filled peppers to the grill. Place on indirect heat and close the lid.
Put parsley, vinegar, garlic, olive oil and salt in a blender or food processor (I used my magic bullet) and process until you have a bright green sauce.
Remove peppers from the grill and serve with salsa verde.
To use the oven instead of a grill, preheat oven to 400° and follow the same steps.You can substitute any grain for the farro, including rice or quinoa. For a gluten-free version, use buckwheat groats or millet.You can substitute mushrooms or tofu for the ground beef for a vegan version.