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 red bell pepper halves, cover, and steam until just tender, 2 to 6 minutes.
Preheat oven to 350 degrees F (175 degrees C).
Heat olive oil in a skillet over medium heat. Cook tempeh and onion until tempeh is crispy, 5 to 10 minutes. Add cannellini beans, spinach, and broth; cook until heated through, about 5 minutes. Remove from heat. Stir feta cheese, nutritional yeast, salt, and pepper into stuffing.
Fill red bell pepper halves with stuffing. Place on a baking sheet.
Bake in the preheated oven, uncovered, until tops are browned, about 30 minutes.