For the farro and sweet potato: Bring a large pot of salted water to a boil. Add the pearled farro and cook to your desired texture, about 10 minutes. Drain, rinse under cold water, drain again and set aside.
Heat the olive oil in a medium skillet over medium-high heat. Add the sweet potatoes, season with salt and pepper and cook, stirring occasionally, until cooked on all sides, 5 to 7 minutes. Remove from the heat and set aside.
For the pickled onions: Combine the vinegar, 1/2 cup water, mustard seeds, black peppercorns, sugar and salt in a medium saucepan over high heat. Bring to a boil and cook until the sugar and salt are dissolved, about 3 minutes. Remove from the heat. Place the onions in a medium heatproof bowl. Strain the vinegar mixture over the onions and let sit until ready to use.
For the vinaigrette: Whisk together the orange juice, apple cider vinegar, whole-grain mustard and honey in a medium bowl until smooth. Slowly whisk in the olive oil and season with salt and pepper. Set aside.
For the build: Combine the cooked farro with the baby kale in a large bowl. Drizzle with 3/4 of the dressing. Season with salt and pepper and toss until evenly coated. Divide into 4 serving bowls.
Divide the sweet potatoes, cucumbers, cherry tomatoes, feta, avocado and 1 cup of the pickled onions evenly among the bowls. Drizzle with the remaining dressing.