Line plate with paper towels and set aside.
Place large skillet on stovetop over medium heat. When pan is warm, add 6 slices bacon and cook, flipping bacon as needed, until crisp.
Transfer cooked bacon to plate lined with paper towels. Set aside. Transfer bacon grease from skillet to heat-resistant bowl or glass jar.
Return 1 tablespoon bacon grease to skillet. Add 1 cup diced onion and stir to incorporate. Sauté onion 3 to 6 minutes, stirring occasionally, until softened.
Once onion has softened, add 4 cloves garlic (chopped) and stir to incorporate. Sauté garlic 30 to 60 seconds or until just fragrant.
Once garlic is fragrant, transfer onion-garlic mixture to Crockpot. Add 2 pounds Yukon gold potatoes and 2 cups chicken stock. Stir to incorporate.
Secure lid on Crockpot and set to desired temperature setting. Cook potatoes until fork-tender, approximately 3 hours on HIGH heat or 5 hours on LOW heat.
Place medium saucepan on stovetop over medium heat. When pan is warm, add 3 tablespoons bacon grease and continue warming pan until grease is liquid.
Once grease is hot, sprinkle ¼ cup flour into saucepan. Vigorously whisk flour into bacon grease until paste is formed.
Cook mixture over medium heat just until lightly golden, then remove saucepan from heat. Slowly add 2 cups whole milk (warmed) and whisk to incorporate. Return saucepan to heat and simmer mixture 2 to 4 minutes, whisking constantly, until mixture has thickened.
Transfer milk mixture to Crockpot. Use potato masher to mash fully-cooked potatoes to desired consistency.
Once satisfied with consistency, add 1 cup shredded fresh cheddar cheese, 1 cup sour cream, 2 teaspoons sea salt, and ½ teaspoon freshly cracked black pepper to Crockpot. Stir until ingredients are fully incorporated.
Replace lid on Crockpot and set temperature to HIGH. Cook 30 minutes on HIGH heat.
After 30 minutes, remove lid and stir soup well. Ladle soup into serving bowls. Top with cooked, diced bacon, shredded fresh cheddar cheese, and diced green onions if desired. Serve warm.