Sprinkle each side of chicken thighs with Italian seasoning, salt, and pepper
In large skillet over medium-high heat, melt butter with olive oil
Add diced onion and garlic and sauté for about 2 minutes
Move onions and garlic to side of the pan and add seasoned chicken thighs
Sauté for about 2-3 minutes on each side until nicely browned
Transfer contents of pan to slow cooker
Add chicken broth
Cover and cook on low for 5-7 hours or high for 3-4 hours
Remove chicken thighs and cover to keep warm
Add cream, mustard, and lemon juice to slow cooker and whisk until combined
To thicken sauce, pour it into a small saucepan and whisk in 1 tablespoon of cornstarch
Bring sauce to a boil for 2-3 minutes until thickened to desired consistency
Add additional salt and pepper to taste
Pour sauce over chicken and serve