Thaw broccoli in the microwave completely, about 4 to 5 minutes. Set aside.
In a large bowl, combine the chopped onion, mayo, cream of mushroom, beaten eggs, and melted butter. Whisk until smooth.
Add the stuffing mix, only 1 Cup of cheddar cheese, and broccoli. Stir with a spoon until mixed.
Scoop mixture into the slow cooker crockpot bowl.
Cook on High Heat with lid on for 1 ½ hour.
Sprinkle the remaining cheese (¼ cup) on the top and reduce heat to Low. Cover and cook 30 more minutes.