Place the pancetta in a large pot or Dutch oven over medium heat. Cook, stirring occasionally, until the fat renders and the pancetta is browned and crispy. Remove with a slotted spoon and set aside.
Add the red onion and cabbage to the rendered fat in the pot. Season with a generous pinch of salt — this helps draw out moisture so the vegetables can brown, not steam. Cook, stirring occasionally, until the onion is soft and the cabbage has crisped and caramelized in spots.
Add the tomato paste. Stir to coat the vegetables and cook for about 1 minute, just until the paste darkens slightly and smells sweet and rich. Don’t let it burn.
Stir in the cooked farro, Borlotti beans (with some of their broth if you have it), and diced potato. Add 2 cups of water or chicken stock. Season with salt and pepper.
Bring to a gentle simmer and cook for about 20 minutes, until the potatoes are tender and the stoup has thickened — you want it somewhere between brothy and stewy, with body and richness.
Taste and adjust seasoning. Add a knob of cold butter and mix it in until it dissolves into the stoup. Add a splash of good-quality balsamic vinegar to brighten the flavor. Ladle into bowls and top each serving with crispy pancetta, a drizzle of great olive oil, and a generous sprinkle of the sage-parsley mixture.