Place bacon slices in a large, cold cast iron skillet over medium heat. Cook, turning occasionally, until crispy, 7 to 10 minutes. Drain bacon slices on paper towels and reserve remaining grease in the pan.

Meanwhile, place diced potatoes in a microwave-safe bowl with some water. Microwave on high for 3 minutes, stir, and heat for 3 minutes more.

Saute onion and garlic in the pan with the bacon fat until lightly browned, about 5 minutes. Add potatoes; cook until lightly browned, 3 to 5 minutes more. Stir in kale. Pour in a spoonful or two of stout and bring to a boil until kale is bright green, scraping the browned bits of food off the bottom of the pan with a wooden spoon.

Season kale with oregano, tarragon, salt, and pepper. Add beans and remaining stout. Bring to a boil. Cover, reduce heat, and simmer until potatoes are fork-tender, about 10 minutes. Drain mixture through a colander set over a bowl; reserve the cooking liquid.

Melt butter in the pan. Whisk in flour until a light golden brown paste forms. Add tomato paste and some of the reserved liquid; whisk until blended. Whisk in the remaining liquid and reduce until sauce is rich and thick, 3 to 5 minutes.

Combine sauce with the vegetable mixture in a serving dish; season with salt and pepper. Crumble the bacon and sprinkle it on top.