To cool cooked farro and other grains more quickly, spread them out on a rimmed baking sheet and transfer to refrigerator until cool.
For the dressing, whisk together grapefruit juice, shallot, and Dijon; season with salt and pepper. Whisk in oil.
Toss lettuces and farro with 1/3 cup dressing and divide among 4 bowls. Top with flaked trout, avocado slices, and grapefruit. Season with pepper, drizzle with more dressing, and serve.