Heat oil in a saucepan over medium-high heat. Saute onion and garlic until onion is almost clear but not browning, about 5 minutes. Add buckwheat; stir until coated, about 1 minute. Add broth and bring to a boil. Simmer until liquid is absorbed, about 15 minutes. Add tomatoes, parsley, red pepper flakes, salt, and pepper. Cook and stir until heated through, about 3 minutes.
Preheat the oven to 350 degrees F (175 degrees C).
Remove tops and seeds from peppers. Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add peppers, cover, and steam until tender, about 5 minutes.
Stand peppers up in a casserole dish and fill them with the buckwheat mixture.
Bake in the preheated oven until browned, about 15 minutes.