1. 1. Preheat the oven to 375°F (190°C).

2. 2. Cook the spaghetti according to package instructions, then drain and set aside.

3. 3. Season the chicken breasts with salt and pepper, then cook in a skillet until fully cooked. Slice the cooked chicken into bite-sized pieces.

4. 4. In a bowl, mix together the low-fat sour cream, diced fresh onion, and diced roma tomatoes.

5. 5. In a large baking dish, layer the cooked spaghetti, sliced chicken pieces, and sour cream mixture.

6. 6. Top with bread crumbs for a crispy topping.

7. 7. Bake in the preheated oven for 20-25 minutes until the top is golden brown and bubbly.

8. 8. Serve hot and enjoy!