Preheat oven to 375 degrees.
Heat 1 tablespoon olive oil in a large oven-safe skillet (such as an enameled cast-iron skillet or sauté pan) over medium-high heat. Add the onion, reduce the heat to medium-low, and cook for 7–8 minutes, stirring often, until softened.
Transfer about 1/3 of the onion to a mixing bowl for the meatballs and let it cool slightly.
Add the remaining 2 tablespoons of olive oil to the skillet. Keeping the heat at medium-low, add the bell peppers and cook, stirring frequently, for 4–5 minutes, or until they begin to soften.
Add 3 cloves of garlic (minced), 2 tablespoons harissa paste, 1 tablespoon cumin, and 2 teaspoons coriander. Cook for 30 seconds, stirring constantly, until fragrant.
Add the crushed tomatoes, bring to a simmer, and cook for 3–4 minutes to let the flavors come together. Keep warm.
To the bowl with the reserved onions, add the remaining 2 cloves of garlic (minced), 1 tablespoon harissa paste, 1 teaspoon cumin, 1 teaspoon coriander, 1 teaspoon salt, 1/4 teaspoon black pepper, panko, egg, ground chicken, and 1/4 cup cilantro. Mix until just combined.
Form into 8 large meatballs and gently nestle them into the tomato-pepper sauce.
Cover with the lid slightly ajar and bake for 25–30 minutes, or until an instant-read thermometer inserted into the center registers 165 degrees.
Sprinkle with remaining cilantro before serving.