Bring steaks to room temperature and season meat liberally with Kosher salt and coarse black pepper. Preheat a large, cast-iron skillet or griddle to medium-high.
Preheat oven to 450°F.
In a skillet or medium saucepot, melt 2 tablespoons butter over medium heat and add onions, garlic, salt, pepper, pimentón and bay. Cook the onions until very soft, about 15-20 minutes, stirring occasionally. Add sugar, vinegar, Worcestershire, mustard, 1/2 cup chicken stock and the tomato sauce. Bring onions to a boil then reduce heat to simmer.
Place potatoes in a pot and cover with water. Bring to a boil, salt the potatoes and cook to tender; drain and put back in hot pot. Add remaining 1/2 cup chicken stock, cheese and remaining 2 tablespoons butter. Mash to desired consistency and season with salt and pepper to taste. Cover to keep warm.
Place the beans and shallots on a baking sheet and drizzle with olive oil. Season with salt, pepper then sprinkle the red peppadews or pimientos over the beans. Roast the green beans to tender-crisp and brown at edges, 15 minutes.
Drizzle steaks with a little olive oil then cook in the hot pan for 8-10 minutes for medium-rare. Cool the steaks then slice 1/4-inch thick against the grain.
Fan the sliced steak on plates and top with some barbecued onions. Serve mashed potatoes and green beans alongside.
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