Put the couscous in a large bowl, add the 2 cups boiling water, 1 teaspoon salt and orange zest, then cover and let steam for 5 minutes. Remove the cover and fluff with a fork.
Whisk together the orange juice, vinegar and olive oil in a small bowl. Add half of the dressing to the couscous along with the onion, orange, mint and parsley and gently combine with a fork.
Put the radicchio in a bowl, add the remaining dressing and season with salt and pepper, to taste. Transfer the couscous to a large platter, top with the radicchio and garnish with slivered almonds.