Preheat oven to 375°F. Trim inexperienced tops from beets; reduce off stems and discard, reserving greens. Gently scrub 4 beets and put aside (reserve remaining beets for one more use). Toss beets with 2 tablespoons oil in roasting pan. Sprinkle with salt. Cowl pan with foil. Bake till beets are tender, about 50 minutes. Let beets stand lined at room temperature 20 minutes. Peel beets and reduce into 1/2-inch cubes.
Whisk crème fraîche, horseradish, lemon juice, and minced shallot in medium bowl. Add beets and toss to coat. Season to style with salt and pepper. Let stand at room temperature not less than 20 minutes. DO AHEAD: Could be made 2 hours forward. Let stand at room temperature.
For flank steak
Whisk 2 tablespoons olive oil, Dijon mustard, Worcestershire sauce, chopped rosemary, minced garlic, and floor black pepper in 13x9x2-inch glass baking dish. Add steak; flip to coat. Cowl and chill not less than 1 hour and as much as 2 hours.
Spray grill rack with nonstick spray. Put together barbecue (medium-high warmth). Take away steak from marinade; sprinkle with salt and pepper. Grill steak to desired doneness, about 5 minutes per aspect for medium-rare. Take away from grill; let stand 5 minutes.
In the meantime, warmth remaining 2 tablespoons oil in heavy giant skillet over medium-high warmth. Add reserved beet greens; sauté till wilted, about 3 minutes. Take away from warmth, cowl, and let stand 3 minutes.