Cook potatoes in a large deep pot with chicken stock for 10-15 minutes or until fork tender.
Meanwhile, add bacon drippings and butter to a 4 quart pot.
Cook onion in drippings over medium-high heat for 2 minutes or until translucent. Add garlic, cook for 1 minute longer. Whisk in flour, stir until flour is absorbed, cook for 1 minute.
Over medium-high heat, whisk half and half into roux. Bring to a boil, then lower heat and simmer while gently bubbling for 5 minutes to thicken. Add seasoned salt, pepper and old bay, if using. Mix well.
Add 2 cups cheese to thickened sauce. Mix until melted. Remove from heat. The sauce will be thick.
Once potatoes are cooked. Use a potato masher to smash to your desired consistency leaving some chunks. Pour cheese sauce into the pot with the smashed potatoes. Mix well.
Simmer on medium-low uncovered for 10 minutes to reduce and thicken. You may adjust thickness further by using a slurry of cold water and flour or cornstarch to your personal taste. You may also think using additional stock to suit your taste.
At the end of cooking, add 1/2 sour cream, bacon and green onions. Reserve bacon and green onion for top
Garnish with remaining cheese, sour cream, bacon and green onions before serving.