In a saucepan, cook butter and flour until the butter is melted and raw flour taste is cooked out. This is called a roux. Cook the roux until it is golden brown.
Add cream and milk. Simmer for 5 minutes, until thickened.
Add cheese and cook for 1 more minutes.
Turn off heat and add spinach leaves. They will wilt in a few minutes.
Add chicken, pasta and crushed red pepper. Stir to combine.