In a small bowl, whisk together 1/4 cup milk, the sour cream, scallions, garlic, lemon zest and juice. Let stand for 5 minutes; season. Cover dressing; refrigerate.
Place the flour and remaining 1 1/2 cups milk in 2 separate medium bowls. Working with 1 steak at a time, season each generously and dredge in the flour to coat, dip into the milk, then dredge in the flour again. Transfer to a sheet of parchment. Reserve remaining milk and flour for the gravy.
In a large heavy skillet (preferably cast-iron), heat 5 tbsp. oil over medium-high heat. Cook 2 steaks, flipping halfway through, until golden, 1 1/2 to 2 minutes; transfer to a baking sheet. Repeat with the remaining 3 tbsp. oil and steaks.
Reduce heat to medium. Sprinkle 3 tbsp. of reserved flour over the drippings in the skillet; whisk for 1 minute. Slowly whisk in 1 cup of the remaining milk. Bring to a boil, whisking; add more milk to reach desired thickness. Reduce heat to low. Cook gravy for 1 minute; season. Spoon the reserved dressing over lettuce; serve with steaks topped with gravy.
