Heat butter in a nonstick saucepan over medium-high heat.
Add onions, garlic and saute for about 2 minutes until garlic is fragrant and onion is partially softened.
Sprinkle flour while whisking constantly until mixture becomes golden-brown in color and smells nutty.
Pour milk, while constantly whisking until sauce is smooth and there are no lumps.
Continue cooking until sauce becomes thick.
Remove from heat.
Add Italian seasoning, salt, pepper, cheddar cheese, mozzarella cheese.
Mix until cheese is melted and sauce is smooth. Keep aside.
Grease slow cooker with oil.
Place half the slices of gold potatoes and sweet potatoes in an alternating pattern in slow cooker (6 quart or larger). They can overlap.
Pour half the cheese sauce on top of the potatoes and spread it as evenly as possible.
Arrange another layer of potatoes on top of the sauce.
Pour remaining cheese sauce on top and spread it out as evenly as possible.
Jiggle the crockpot slightly so that the sauce spread out further along the sides too. Do not mix!
Cover and cook on High heat for 3-4 hours or until the potatoes are tender.
Optional step: My mom does this but I usually skip this step. Spread a few sheets of paper towels or a clean thin dish towel on top of the crockpot. It should be overhanging on the sides of the crockpot. Then, put the lid. The towel will absorb excess moisture and prevent "soupy potatoes".
In the final 15 minutes, sprinkle parmesan cheese on top. Also, sprinkle fresh parsley.
Cover and continue cooking until cheese is melted. Enjoy!