Cook farro per package directions. Heat grill to medium-high.
Meanwhile, in large bowl, toss peppers with 1 teaspoon oil and a pinch each of salt and pepper. Grill until slightly charred, 2 to 3 minutes per side. Meanwhile, grill scallions until charred, 1 to 2 minutes per side. Transfer to cutting board and cut peppers into pieces. Thinly slice 1 inch of each scallion white, then cut remaining greens into 2-inch pieces.
In same large bowl, whisk together vinegar, cumin, coriander, red pepper flakes, remaining 3 Tbsp oil, and ½ tsp each salt and pepper. Add tomatoes, peppers, sliced scallions, and farro and toss to combine. Fold in remaining scallions.
