Heat your grill to high. Toss the shrimp in a bowl with 2 tablespoons of olive oil and 1/2 teaspoon each of salt and pepper. Grill the shrimp for 1 to 2 minutes per side or until just cooked through. Transfer shrimp to a clean medium bowl.
Heat 1/3 cup olive oil in a medium saucepan over medium heat. When oil is hot, add the onion, garlic, and serrano chiles and cook until soft. Add the cumin and mustard seeds and cook for 1 minute. Add the vinegar, bay leaves, and cilantro and bring to a boil. Remove from the heat and immediately pour over the shrimp and stir to coat. Let cool to room temperature, cover, and then refrigerate for at least 8 hours or overnight, stirring a few times during that time. Transfer the escabeche, with a slotted spoon, to a platter and garnish with sliced radishes and torn cilantro.