Preheat the broiler. In a medium pot of boiling, salted water, parboil the potatoes for 5 minutes; drain.
Pat the beef dry; season with the kosher salt and pepper. Toss with 4 tbsp. EVOO and the garlic and rosemary. Thread as many pieces as will fit on each skewer.
In a large nonstick skillet over medium-high heat, melt the butter with the remaining 1 tbsp. EVOO. Add the potatoes; season with salt, coarse black pepper and paprika. Cook, stirring occasionally, until browned, 10 to 15 minutes.
Broil the meat on the top rack, turning occasionally, until cooked, about 10 minutes.
In a bowl, make the spiked ketchup: Stir the ketchup with the Worcestershire, horseradish, Tabasco and a pinch of coarse pepper.
Squeeze the lemon over the beef and serve with the greens, crispy potatoes and spiked ketchup.