In the bottom of a 6-quart slow cooker, mix together the cream of mushroom soup, milk, onion powder and ground pepper. Stir until all ingredients are well combined.
Add in green beans, parmesan cheese, cheddar cheese and half of the french fried onions. Stir gently until well combined.
Cook on low for 2 hours, stirring occasionally. When cooking time is complete, stir mixture well and add in the remaining french fried onions and cook for an additional 20-30 minutes.
After the additional 20-30 minutes is up, remove the dish from the slow cooker and serve immediately.
Notes
Other soups can be used instead of cream of mushroom, see my tips above.
Freshly shredded/grated cheese works best. You can play around with your cheese preferences or omit completely.
Frozen green beans will work, I suggest defrosting before using.