Preheat the oven to 400 degrees F. Pierce the potatoes with a fork and brush with olive oil. Bake directly on the oven rack until tender, about 1 hour. Cool slightly, then cut into chunks. Mix the sour cream, buttermilk, chives and lemon juice in a large bowl and season with salt and pepper. Stir in the potatoes.
Meanwhile, preheat a grill to high. Brush the steak and onion rings with olive oil and season with salt and pepper. Grill the steak until marked on the bottom, about 7 minutes, then turn and cook 4 to 5 more minutes for medium-rare. Grill the onion rings until charred, about 2 minutes per side. Transfer the steak and onion rings to a cutting board and let rest 5 minutes.
Roughly chop the onion rings and toss with the spinach, steak sauce, 3 tablespoons olive oil, and salt and pepper to taste in a bowl. Thinly slice the steak. Divide the salad among plates, top with the steak and serve with the potatoes.