Preheat the oven to 400 degrees F.
Place each beet on an individual piece of aluminum foil. Drizzle the beets with 1 tablespoon olive oil and season with 1/2 teaspoon kosher salt and 1/2 teaspoon black pepper. Fold into loose packets on a sheet pan. Roast the beets for 45-50 minutes until they can be easily pierced with a knife.
When the beets are cool enough to handle, use a towel to rub off their skins. Dice the beets into 1/2-inch pieces.
Combine the farro and water in a medium saucepan. Bring the water to a boil. Reduce the heat and simmer the farro for 12-15 minutes until it is cooked through but still has a chewy bite. Drain the farro and remaining water from the saucepan into a fine mesh strainer. Rinse with cold water.
For the vinaigrette, whisk the garlic, mustard, salt, pepper, lemon juice, white wine vinegar and olive oil in a small bowl.
In a large bowl, combine the farro, red onions, arugula, walnuts, feta and parsley. Drizzle with half the vinaigrette. Toss to combine.
Gently fold in the diced beets, adding more vinaigrette as desired.