Preheat oven to 400°F / 200°C.
Cut the stems and tops off the bell peppers. Scoop out and discard the seeds and membranes. Arrange the bell peppers upright in a baking dish and sprinkle with some salt.
Bake peppers until they are starting to turn tender, 15 minutes. (The edges should start to brown slightly and tilt inward.)
While peppers start roasting, combine couscous, lentils, harissa, paprika, tomato, and garlic. Stir well. Gently fold in feta.
Taste filling and season it with some salt and / or pepper, if needed.
Fill peppers with couscous-lentil mixture.
Return filled peppers to oven and continue baking until filling is heated and peppers are soft, 15 to 25 minutes more (this will vary depending on the thickness of the bell peppers).
Squeeze lemon juice over finished peppers and top with fresh parsley.
Serve stuffed peppers warm with yogurt for topping.