Melt butter in a medium saucepan, add shallots and garlic and saute until soft. Add rum, bring to a boil and reduce to 1/3 cup. Add stock, bring to a boil and reduce the heat. Whisk in remaining ingredients and simmer until reduced to 2 cups. Season with salt and pepper to taste. Serve at room temperature.
Preheat grill. Brush each steak with olive oil and season with salt and pepper to taste. Grill for 3 minutes on each side for medium-rare doneness. Drizzle with Rum Sauce.
Place warm potatoes, peppers, onion and cilantro in a large bowl and toss to combine. Whisk together vinegar, olive oil and salt and pepper. Pour vinaigrette over potatoes and season again with salt and pepper to taste.