Wash 1 bunch rainbow chard well; drain in a colander. Tear leaves from stems, keeping stems and leaves separate. Bundle leaves and cut them into wide strips. Trim stems, remove the strings, and slice about 1/4 inch thick (about 8 cups leaves and 1 cup stems).
In a large skillet, cook stems, garlic, za'atar, and salt and black pepper (to taste) in hot oil over medium heat for 2 to 3 minutes or until the stems begin to soften. Increase heat to medium-high; add chard leaves. Cook and stir 2 to 3 minutes or until tender. Finish with a squeeze of fresh lemon juice. Top with toasted pepitas.