Preheat the oven to 400°F (200°C).
Cook the spaghetti according to package instructions until al dente.
While the pasta cooks, dice the chicken breast into bite-sized pieces and season with salt, pepper, and Italian seasoning.
In a large oven-safe skillet, melt 2 tablespoons of garlic butter over medium heat.
Add the diced chicken to the skillet and sauté until browned and cooked through, about 5-7 minutes.
While the chicken cooks, chop your mixed vegetables and toss them in the remaining garlic butter along with a sprinkle of salt and pepper.
After the chicken is cooked, push it to one side of the skillet and add the prepared vegetables to the empty side.
Transfer the skillet to the preheated oven and roast for 15-20 minutes until the vegetables are tender and slightly caramelized.
Once the veggies are done, add the cooked spaghetti to the skillet and toss everything together.
Sprinkle the shredded mozzarella cheese over the top and return the skillet to the oven for an additional 5 minutes to melt the cheese.
Remove from oven, sprinkle with fresh herbs if desired, and serve warm.