Season chicken with ½ teaspoon salt and ½ teaspoon pepper and cut into 1 inch chunks. In a large skillet, melt 1 tablespoon butter and cook chicken for 5 minutes, stirring occasionally. Remove chicken from pan and set aside.
Add remaining 2 tablespoons butter to the pan and melt over medium heat. Add onion, mushrooms and garlic and cook for approximately 3 minutes or until veggies begin to soften.
Cook and mash butternut squash. Add flour and cook, stirring often for 4 minutes. This is important to cook the flour so that the flour taste won't linger. Add squash, cream and stock, italian seasoning, lemon juice and remaining ¼ teaspoon salt and pepper; continue cooking for 6 minutes.
Remove pan from heat and stir in cheese. Serve over hot linguini.