Into a large, deep skillet over medium-high heat, add vegan butter. When butter is melted, add onions. Saute 2-3 minutes, until softened.
Over the onions, sprinkle all-purpose flour, sea salt, ground black pepper, and garlic powder. Stir until evenly coated.
Add vegan chicken-style broth. Reduce heat to low and simmer, stirring frequently for about 3 minutes, until thickened slightly. Remove from heat.
Stir in vegan sour cream and 2 cups of Cheddar cheese and 1/4 cup Parmesan cheese.
Into the skillet, add the cubed potatoes and stir to coat with cheesy sauce.
Prepare a 6-quart slow cooker with non-stick cooking spray or liner.
Add coated potatoes, cover, and cook on low for 4 hours, or until cooked through.
Add remaining 1/2 cup vegan Cheddar cheese and remaining 1/4 cup vegan Parmesan cheese. Recover and allow cheeses to melt for about 15 minutes.
Garnish with chives and serve hot.