Preheat oven to 425°F.
Toss cauliflower florets with oil and salt and spread evenly on a baking sheet. Bake 30 minutes, tossing once after 20 minutes.
Meanwhile, place 1/2 cup water and cornstarch in a small bowl and whisk to combine. Set aside.
Heat tamari (or soy sauce), brown sugar, garlic, Hoisin, and rice vinegar in a small saucepan over medium heat. Cook until sugar dissolves; about 1 to 2 minutes, stirring often. Add the cornstarch mixture and bring sauce to a boil, whisking often to avoid scorching on the bottom of pan. Reduce to medium-low and simmer until sauce is viscous and thickened, about 2 to 3 minutes. Remove from heat.
Remove cauliflower from oven and brush with half of teriyaki sauce. Place back in the oven and bake until cauliflower is caramelized; about 7 to 10 minutes.
To prepare bowls, divide teriyaki cauliflower, farro (or brown rice), edamame, and red cabbage evenly between each of four bowls. Spoon remaining teriyaki sauce evenly over bowls and top with sliced green onion. Add additional garnishes of choice (toasted sesame seeds, avocado, etc.), if desired.