Wash the mushrooms but keep whole. Cut the onion into thick slices and place in the bottom of the slow cooker, along with the mushrooms. Mince the garlic and add to the slow cooker. Sprinkle on the rosemary then add the balsamic vinegar and broth.

Generously salt and pepper the chicken thighs. Layer them over the onion and mushrooms.

Place on lid and cook on low for 6-8 hours or high for 3-4 hours.