First, set the Greek yogurt and frozen vegetables on the counter to reach room temperature. This prevents curdling and helps the soup blend smoothly. Dice your onions, celery, and garlic while the potatoes cook.
Boil the peeled, cubed potatoes in salted water until tender, about 20 minutes. Drain, mash half with butter or ghee, and season lightly with salt and pepper. Set the remaining potatoes aside to add texture later.
In a large stockpot, brown your chosen ground meat while breaking it up with a wooden spoon. Drain excess grease, then season lightly with salt and pepper.
Stir in onions, celery, garlic, Italian seasoning, and thyme. Cook for a few minutes until fragrant, then add the thawed frozen vegetables.
Pour in the bone broth and bring to a boil. Reduce heat and simmer for 10 minutes. Stir in the mashed and remaining whole potatoes, then simmer for another 5 minutes.
Remove from heat and stir in Greek yogurt or sour cream until fully incorporated. Taste and adjust seasoning. Garnish with cheddar, parsley, or green onions before serving.