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Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce.
Peel and mince garlic. Trim and halve zucchini lengthwise; cut crosswise into ½-inch-thick half-moons. Strip thyme leaves from stems until you have 1½ tsp (1 TBSP for 4 servings); mince leaves. Zest and quarter lemon.
In a small bowl, whisk together ¼ cup water (1⁄3 cup for 4), jam, stock concentrate, minced thyme, and half the mustard (all for 4).