Start by making the marinade for shrimp. Roughly chop, the serrano peppers, garlic,oregano and thyme. Transfer to the blender, add lime juice,1/4 cup olive oil, 1teaspoon salt and 1/2 teaspoon pepper. Blend until smooth, taste for salt. In a glass bowl, combine shrimp and marinade, stir well to combine, cover and refrigerate until ready to use.
Preheat oven to 425 degrees F. Lay the poblano peppers on baking sheet and drizzle with a little oil. When oven is up to temperature, roast the poblanos for 12 to 15 minutes per side. Remove from oven and cover with a clean kitchen towel, set aside.
In a large sauce pan, heat 3 tablespoons of olive oil to medium heat, add the shallots and serrano peppers, cook for 3 minutes. Add the garlic and cook for 1 more minute.Add the remaining ingredients for warm salsa, stir well to combine. Taste for salt and continue cooking until the tomatoes and tomatillos break down and salsa thickens slightly. Cover and keep warm.
Remove the shrimp from refrigerator. Preheat your broiler for 10 minutes. While you are waiting, peel the blistered skins from the poblano peppers, carefully slit open, remove seeds, then fill each one with 1/2 cup of shredded cheese. Transfer the filled peppers to a foil lined baking sheet, set aside.
In a large saute pan, heat 2 tablespoons of olive oil to medium/high heat. At this time, you want to transfer the peppers to the broiler. Add the shrimp to the saute pan and cook for 5 to 6 minutes or until shrimp is pink. The peppers should be ready to pull out of the broiler about then too. You want to make sure your cheese is melted and lightly browned.
To plate, add about 1/3 cup of warm salsa,lay a chile relleno onto salsa, then top with serrano shrimp. Garnish with fresh cilantro lime wedges and caramelized onions. Yields 4 servings.