Preheat oven to 350°F.
Add bacon to a Dutch oven over medium heat.
Cook, stirring occasionally, until crispy, about 10 minutes.
Remove bacon with a slotted spoon; place bacon on a paper towel.
Discard all but 2 Tbsp. bacon grease from pan (reserve extra for another use).
Add onion to bacon grease in pan. Cook, stirring occasionally, until softened, about 5 minutes. Add butter to pan; let melt.
Stir flour into butter and onions. Cook, stirring, 2 minutes.
Stir in 1 ¼ cups milk until combined. Remove from heat.
Stir in 1 ½ cups cheese, ½ cup sour cream, and ½ cup scallions.
Stir in half of frozen tater tots; transfer mixture to a 13x9-inch baking dish. Sprinkle with remaining tater tots, bacon, and cheese.
Bake until bubbly, about 35 minutes. Let cool 5 minutes.
Stir together remaining ¼ cup sour cream, 2 tablespoons milk, chives, dill, and salt, and pepper in a small bowl until smooth.
Sprinkle remaining ½ cup scallions over casserole; drizzle with homemade ranch.