Heat butter in a large pan over medium high heat.
When butter is foamy and melted, add chicken thighs and cook on medium high until golden brown on each side, about 4 minutes per side (add extra minute per side if bones still in thigh).
When chicken is browned and crisp, remove and set aside on plate lined with paper towel.
Add onion to pan and cook to browned, stirring often, about 5 minutes.
Add garlic and stir to prevent burning. Cook until soft and fragrant.
Add stock and let reduce, another 4 minutes.
Turn off heat and add cream, whisking vigorously as you add to pan.
Add chicken back to pan.
When cream is well incorporated into sauce, return heat to medium high.
Stir in lemon juice, parmesan cheese, herbs, red pepper flakes, lemon slices, and asparagus.
Cook 5-8 minutes until chicken has reached 175 internal temperature and asparagus is soft and bright green.
Top with additional parmesan cheese if desired.
Serve immediately over pasta, rice, or with grilled vegetables.