Add onion, garlic, olive oil, dry mustard, thyme, 1/2 teaspoon of salt, and 1/2 teaspoon of pepper to the slow cooker, stirring to combine.

Season chicken with salt and pepper then nestle into slow cooker.

Cover and cook until chicken is tender, 4 to 5 hours on low.

Transfer chicken to serving dish and tent with aluminum foil to keep warm.

Switch slow cooker to high. Stir Swiss chard and whole-grain mustard into liquid in slow cooker and cook until chard is wilted, about 20 to 30 minutes.

Return chicken and any accumulated juices to slow cooker. Season with salt and pepper to taste.

Let chicken heat through (about 5 minutes) then serve.