Heat oven to 450°F. On rimmed baking sheet, toss onions, fennel, and coriander with oil and a pinch each of salt and pepper. Roast 15 minutes.
Season shrimp with sumac and ⅛ teaspoon each salt and pepper. Toss tomatoes into fennel mixture, then nestle in shrimp and lemon halves and roast until shrimp are opaque throughout, 5 to 7 minutes more.
Transfer half each of lemon, roasted vegetables, and shrimp to airtight container for lunch the next day.
Transfer remaining shrimp and lemon to plate. Scatter spinach over remaining vegetables and roast until barely wilted, 1 to 2 minutes. Serve with roasted shrimp and lemon.
