Preheat the oven to 425 degrees. Brush a foil-lined baking sheet with 1 tbsp. EVOO; roast the peppers, cut side up, until just softened, 10 to 15 minutes.
Meanwhile, in a large skillet, cook the onion in 2 tbsp. EVOO over medium, stirring often, until translucent, about 4 minutes. Add the zucchini and garlic and cook, stirring occasionally, until zucchini begins to soften, about 3 minutes; transfer to a bowl. Increase the heat to medium- high and add the lamb and oregano to the skillet; season with salt and pepper. Cook, breaking the meat up with a spoon, until browned, 4 minutes. Off heat, stir in the zucchini mixture, marinara, couscous and 1/2 cup feta; season.
Spoon filling into the pepper halves, top with remaining feta, and bake until peppers are tender, about 10 minutes.