Preheat your oven to 350°F (175°C) and grease a large casserole dish.

Cook the boneless, skinless chicken breasts in a large pot over medium heat for 6-7 minutes on each side until golden brown and cooked through.

In a large pot, bring salted water to a boil and cook spaghetti for 8-10 minutes until al dente. Drain and cool slightly.

In a mixing bowl, combine diced chicken, cooked spaghetti, cream of chicken soup, chopped bell peppers, seasoning, and half of the shredded cheddar cheese.

Transfer the mixture to the greased casserole dish and top with remaining cheddar cheese.

Bake in the preheated oven for 25-30 minutes until bubbly and golden brown on top.

Let the casserole sit for 5 minutes before serving, then garnish with sliced green onions.