Prepare the Potatoes: Peel and dice the russet or Yukon gold potatoes into even chunks. Place in the crockpot with white wine or apple cider, minced garlic, and salt. Cook on high for 4 hours or low for 6-8 hours until tender.

Drain and Dry: Once tender, drain excess liquid and let potatoes rest in the crockpot for about 10 minutes.

Mash the Potatoes: Mash the potatoes until smooth with a potato masher or hand mixer, scraping sides to incorporate flavor.

Incorporate the Creaminess: Add the salted butter and heavy cream, mixing until creamy. Season with salt and pepper to taste.

Add the Cheeses: Fold in Gruyère and Raclette cheeses, covering the crockpot and allowing cheeses to melt on warm setting for about 15 minutes.

Serve Warm: Stir, transfer to a serving dish, and garnish with fresh thyme or sage if desired. Serve immediately.