Pre-heat oven to 400 degrees and line two baking sheets with foil.
Place butternut squash and parsnip cubes in a large resealable ziplock bag along with olive oil and Lebanese Spice Blend. Seal bag and toss well.
Spread vegetables in a single layer evenly between both pans. Bake for 15-20 minutes or until golden brown.
Remove roasted vegetables from both pans and place in a large soup pot. Stir in chicken broth, yogurt and lemon juice. Puree until smooth using an immersion blender. Alternatively, you can puree in a high speed blender then return to a soup pot to warm.
Check for seasoning and adjust accordingly. Sprinkle each bowl with za'atar and another dollop of plain yogurt, if you wish, before serving.