Meanwhile, as farro cooks, arrange pine nuts on a parchment-lined baking sheet and toast in the oven for about 5-7 minutes, or until lightly golden. Watch carefully - they burn quickly! Set toasted pine nuts aside.
Increase oven heat to 400 degrees F.
Arrange pepper halves on a baking sheet and brush with 1 tablespoon olive oil. Season with salt and pepper. Bake in the oven until tender, about 20-25 minutes.
Meanwhile, measure out 1½ heaping cups of frozen chopped spinach, and place the frozen spinach in a heat-safe container. Microwave until thawed. When cool enough to handle, transfer thawed spinach to a fine mesh strainer. Arrange the strainer over a large mixing bowl and squeeze spinach, wringing dry, until all excess water is drained from the spinach.
When farro is fully cooked and tender and all of the broth is absorbed, add the toasted pine nuts and chopped spinach. Add feta and stir until well combined. Season to taste with salt and pepper.
Stuff cooked bell pepper halves with heaping spoonfuls of farro mixture. Serve.