Step 1Heat a grill over medium-high heat. In a medium bowl, toss the shrimp with 1 tablespoon olive oil, the garlic, one-fourth teaspoon salt and a pinch of black pepper. Allow to marinate while the tomatillos, onions and chile are grilled.
Step 2In another medium bowl, toss the tomatillo wedges with just enough olive oil to coat, and season to taste with salt and pepper. Grill the wedges until charred and crisp-tender, about 2 1/2 minutes on each side. Place them in a large bowl, and set aside in a warm place.
Step 3Lightly brush the green onions and the serrano chile with a little olive oil. Grill until tender and charred on all sides, about 2 minutes. Remove from the heat and set aside until cool enough to handle. Slice the onions crosswise into 2-inch pieces and add to the tomatillos. Halve the chile and remove the seeds, then chop and add to the tomatillos. Toss and season to taste with salt and pepper.
Step 4Grill the shrimp just until cooked through (they will be opaque, firm and pink), 1 1/2 to 2 minutes on each side. Remove the shrimp from the grill and toss them in the bowl with the tomatillos along with the cumin and oregano. Serve with grilled tortillas.