In a large sauté pan, heat olive oil and ghee until shimmering.
Add the slices of leek and sauté until softened, stirring occasionally, about 5 minutes.
When you can easily pierce the leek slices with a fork
add the couscous.
Toast the couscous until it smells fragrant and you notice the couscous turns a light brown. This should take about 5-8 minutes.
Add the garlic, give it all a stir and, as soon as you smell the garlic (about 1-2 minutes), add the broth and butter.
Bring to a boil, then simmer, covered, for about 15-20 minutes. The couscous is done when the liquid is absorbed and the couscous tastes al dente. You want it to have a little bit of a bite, just like pasta.
Take the pot off the heat, use a fork to fluff the couscous and stir in fresh herbs and lemon zest.