Sprinkle a small amount of salt and lemon juice over chicken and allow to stand at room temperature for 30 minutes. In a preheated 350 degree oven, toast coriander and cumin on a baking sheet for 5 minutes. Allow spices to cool and crush in a blender or with the bottom of a heavy pot. Combine coriander and cumin with ginger, garlic, cayenne, and yogurt to make a marinade. Dip chicken breasts into yogurt mixture and marinate, refrigerated for 24 hours. Turn occasionally. Broil marinade-coated chicken 5 inches from heat source, about 8 minutes. Turn and broil other side until tender, 6 to 8 additional minutes. Cover the serving plate with a thick layer of julienned radishes, place 2 pieces of chicken on top, and garnish each plate with four pieces of scallion and two pieces of lime.