For the farro: Bring 4 cups water to a boil and add the farro and salt. Bring to a simmer and simmer until cooked, about 15 minutes. Drain and let cool.
For the scallion pesto: Grill the scallions until charred, 4 to 6 minutes. Add the scallions to a blender with the basil, vinegar, salt and pepper and process. With the motor running, slowly add the olive oil and mix until the consistency of pesto.
For the vegetables: Drizzle the zucchini and squash with olive oil and sprinkle with salt and pepper. Grill on medium heat, about 5 minutes each side. Set aside to cool, then cut into 1/2-inch pieces.
Grill the red peppers on medium heat, about 5 minutes each side. Place in a bowl and cover in plastic wrap to steam for about 8 minutes. Peel off the skins, remove the seeds and cut into 1/2-inch strips.
Combine the farro, zucchini, squash, peppers and basil in a large bowl. Toss with the scallion pesto and season to taste with salt and pepper.
For the scallops: Drizzle the scallops with olive oil and sprinkle with herbes de Provence, salt and pepper. Grill over medium heat, 3 to 4 minutes per side.