Step 1Heat the oil in a large saucepan over medium heat. Add the onion and bell pepper, season with salt and cook, stirring occasionally, until a little tender, about 5 minutes.
Step 2Add the garlic, paprika and a pinch of salt and stir well for 1 minute. Raise the heat to high and stir in the farro. Add the vinegar and cook, stirring, until the acidic smell burns off. Add the tomato juice and 1 ¼ cups cold water. Cover and bring to a boil. Reduce the heat to low and simmer, stirring and scraping the bottom of the pot occasionally, until the farro is tender and the water is absorbed, 20 to 25 minutes.
Step 3Remove from the heat and let sit covered for 10 minutes. Gently fold the farro to evenly mix all the ingredients, then let cool to room temperature.
Step 4Divide the farro among bowls and top with the tuna, olives, radicchio, cucumber and parsley. Drizzle with olive oil, then splash with a little vinegar and sprinkle lightly with salt before serving.