To make avocado ranch sauce: In bowl, whisk mayonnaise, sour cream, buttermilk, ranch seasoning, salt and pepper until smooth. Fold in green onions and diced avocados; set aside.
To make burgers: In small bowl, whisk ranch seasoning, paprika, salt and pepper.
Heat large cast-iron pan or griddle over high heat. Divide meat into four 6-ounce patties and make deep depression in centers with thumb. Season each with spice rub on one side.
Add canola oil to pan and heat until shimmering. Place on burgers in pan, spice side down, and cook until golden brown and lightly charred, 4 minutes. Turn burgers over and cook 4 minutes, or until they reach desired doneness.
Top each burger with two slices cheese. Add water to pan and cover tightly with lid 10 seconds, or until the cheese is melted.
Place burgers on bottom buns and top each with avocado ranch sauce and top buns.