Bring a large pot of salted water to a boil and add your pasta, cooking until al dente (about 10-12 minutes). Drain and set aside, reserving 1 cup of pasta water for later sauce adjustments.
While pasta cooks, heat 2 tbsp olive oil in a large skillet over medium-high heat. Season chicken pieces with salt and pepper, then cook until golden brown on all sides (6-8 minutes). Transfer to a plate and set aside.
In the same skillet, melt 3 tbsp butter and add diced onion, cooking until softened about 3 minutes. Add minced garlic and red pepper flakes, stirring constantly for 30 seconds until fragrant.
Add sliced mushrooms to the pan and sauté for 4-5 minutes until they release their moisture and begin to brown. This concentrates their earthy flavor and creates a richer sauce base.
Pour in 1 cup chicken broth and 2 cups heavy cream, stirring gently to combine. Add warm hearty creamy pasta's key flavor component—1 tsp Italian seasoning—and bring the mixture to a gentle simmer over medium heat for 5 minutes.
Return the cooked chicken to the skillet along with fresh spinach, stirring until the spinach wilts completely (about 1-2 minutes). The creamy pasta sauce should coat the chicken beautifully and keep the spinach tender.
Add the cooked pasta to the sauce and toss gently to combine, ensuring every noodle gets coated. Sprinkle 1/2 cup grated parmesan cheese over top and stir until melted and creamy.
If the sauce seems too thick, add reserved pasta water a splash at a time until reaching your desired consistency. Taste and adjust seasonings before serving immediately.