Place cumin and coriander in a small nonstick pan over medium heat; cook 1 minute or until toasted, stirring frequently. Place spice mixture in a spice or coffee grinder; pulse 3 times or until coarsely ground. Combine ground spices, 1/4 teaspoon salt, paprika, 1/4 teaspoon black pepper, and crushed red pepper in a small bowl, stirring with a whisk. Spread spice mixture onto a large plate; lightly roll each skewer in spice mixture.
Combine remaining 1/4 teaspoon salt, remaining 1/4 teaspoon black pepper, onions, lemon juice, oil, and garlic in a small bowl; stir well with a whisk.
Place skewers on grill rack coated with cooking spray. Grill 8 minutes or until done, turning occasionally. Arrange skewers on a platter; drizzle with juice mixture. Serve with lemon wedges, if desired.