In bowl toss shrimp with 1 tablespoon olive oil, garlic, 1/4 teaspoon salt and pepper. While vegetables are cooking - let shrimp marinate.
In another bowl combine tomatillo wedges with olive oil to coat and season with salt and pepper. Grill wedges until charred and crisp-tender (5 minutes, turning once). Place in a large bowl and set aside.
Brush green onions and serrano chile with olive oil. Grill until tender and charred (2 minutes). Remove from heat and let cool - then slice onions crosswise into 2-inch pieces and add to the tomatillos. Toss and season with salt and pepper.
Grill shrimp (4-5 minutes, turning). Remove from grill and toss in bowl with vegetables along with the cumin and oregano.