In a large sauté pan, heat olive oil and ghee over medium heat until shimmering.
Add leek and tomatoes and sauté until softened, stirring occasionally, about 5 minutes.
When the tomatoes start to break down and the leeks soften, stir in the couscous.
Stirring often, toast the couscous until it smells fragrant and you notice it turns a light brown. This should take about 5-8 minutes.
Add the garlic and za'atar and stir to incorporate into the other ingredients. As soon as you smell the garlic (about 1-2 minutes), add the broth and butter.
Bring to a boil, then lower the heat and 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 some bite similar to pasta.
Take the pot off the heat, use a fork to fluff the couscous, stir in fresh herbs and lemon zest.
Serve immediately.