Arrange rack in middle of oven; preheat to 400°F on Roast setting. If your oven doesn’t have Roast, use Bake setting.
Mix garlic, cumin, oil, paprika, pepper, oregano, 1/2 tsp. salt, lemon zest and juice in a medium bowl.
If spatchcocking the chicken yourself: Place chicken breast side down on work surface. Spatchcock chicken by cutting along both sides of backbone with kitchen shears. Remove backbone; reserve for stock if so inclined. Turn chicken breast side up and splay open. Press down on breastbone with palms until you hear it crack and chicken is as flat as possible. Pat chicken dry with paper towels.
Using your fingers gently loosen skin from breasts and thighs, being careful not to tear skin. Continue using your fingers to gently spread 2 heaping tablespoons of spice mixture under the skin (reserve remaining spice mixture), then season chicken all over with remaining 1 teaspoon salt. Transfer chicken, spread flat and skin side up, to a roasting pan or large oven-proof skillet.
Roast chicken 20 minutes. After 20 minutes, begin brushing the chicken in 10 minute intervals with the reserved spice mixture and pan juices. Approximate total cook time is 50 minutes or until juices run clear when thigh is pierced with a fork or an instant-read thermometer inserted into the thickest part of the thigh registers 165ºF. (If your chicken needs more time in the oven, do it in 5-minute increments to avoid over-roasting.)
While chicken is roasting, put cilantro, jalapeños, garlic, oil, lime juice, and salt into a blender or food processor and process until combined. Add mayonnaise and process until well blended. Transfer to a small bowl, cover, and chill until ready to use.
Let chicken rest for 10-15 minutes.
Then transfer chicken to a platter and serve with the Cilantro-Lime Mayo. Garnish with cilantro and lime slices, if desired.