Using a mortar and pestle, coarsely crush coriander seeds. Stir in thyme, kosher salt, MSG, garlic, and pepper. Set aside.
Place chicken, breast side down, on a large rimmed baking sheet. Using kitchen shears, cut along either side of backbone; remove backbone. Flip chicken breast side up; press on breastbone to flatten chicken.
Pat both sides of chicken dry with paper towels. Drizzle underside of chicken with 1 tablespoon oil, and sprinkle with about 2 tablespoons spice mixture and 1 ½ teaspoons lemon zest. Flip chicken breast side up; drizzle skin with remaining 2 tablespoons oil, and sprinkle with remaining spice mixture and remaining 1 ½ teaspoons lemon zest. Refrigerate chicken, breast side up, uncovered, on a large rimmed baking sheet, at least 1 hour or up to overnight (12 hours).
Remove chicken from refrigerator, and let stand at room temperature 30 minutes. Meanwhile, preheat oven to 425°F.
Roast chicken on baking sheet in preheated oven until an instant-read thermometer inserted in thickest portion of thighs registers 155°F, 45 to 50 minutes. Let rest 10 minutes. (Temperature will continue to rise to 165°F.) Garnish with thyme sprigs and lemon slices.
