Coarsely chop the onion. Mince the garlic cloves. Rinse the herb(s).
Heat the oven to 400ºF. Place the 1 cup farro in a baking dish and toast in the oven for 10 to 15 minutes.
In a wide, deep pan (the all-in-one pan works great) heat the 2 tbsp olive oil over medium-high heat. Add the onion and cook until softened (5-7 min), seasoning with salt. Add the garlic and red pepper flakes and cook for 1-2 minutes. Add the tomato purée, herbs/sprigs and 3 cups broth or water (or a combination of both). Season with salt and pepper and bring to a boil.
Add the lentils and farro and stir completely. Bake in the oven for around 25-30 minutes, stirring halfway through. Check in the middle of cooking to ensure it doesn’t get too dry, and if it does add ¼ cup boiling water and stir well.
Cut the feta into thin (less than ¼”) slices. Once the farro is cooked (so it’s chewey but not crunchy), remove the dish from the oven and turn on the broil setting to high. Stir the lentils and farro once more, discard the herb sprigs, then top with the slices of feta. Broil until the feta is molten and brown in spots, 6 to 8 minutes.