Fully thaw the turkey according to the instructions on the packaging
Remove netting and lay it flat on the bottom of your slow cooker.
Cut your onion into rings and chop garlic and arrange them around the turkey.
Mix the marinade. In a medium-sized bowl, mix Herbs de Provence, dijon mustard, kalamata olive brine, and lemon juice. Rub the sauce over the top of the turkey.
Slice butter and arrange butter on top of the turkey
Cover and slow cook on high for 5 hours or on low for 7-8 hours. Use a meat thermometer and check the internal temperature. It should reach 165 degrees before it is safe to eat.
Remove the turkey from the slow cooker. Place all the drippings in a pan and add a cornstarch slurry to make gravy.