ANCHOVY BREADCRUMBS: Heat oven to 325°F. Melt butter over medium heat in a small skillet. Add anchovies and cook, stirring, until they break down, about 1 minute. Stir in parsley, lemon zest, garlic, and breadcrumbs and transfer to a rimmed baking sheet. Bake until crumbs are golden brown and crisp, about 2 minutes. Set aside.
BURRATA CREAM: Combine burrata, lemon juice, olive oil, and milk in a blender and puree until smooth. Season to taste with salt and pepper and set aside.
FLANK STEAK: Season steak generously with salt and pepper and let sit at room temperature for 30 minutes.
Melt 3 tablespoons butter in a large cast-iron skillet over medium heat. Add steak and cook for 2 to 3 minutes on each side (or to desired doneness), turning once. Transfer to a plate and tent with foil to keep warm.
Lower heat to medium-low and add remaining 3 tablespoons butter. Once melted, add scallions and cook until softened, 2 to 3 minutes. Add radishes and rosemary and cook until radishes are tender, 5 to 6 minutes. Season to taste with salt and pepper.
TO SERVE: Thinly slice steak against grain and divide between 2 plates. Top each with sautéed vegetables and some butter from skillet. Garnish with burrata cream and anchovy breadcrumbs. Serve immediately.