1In a large sauté pan heat the oil and butter over medium-high heat. Once the butter has melted and the oil is hot, toss in ¾ cup of the onions and a pinch of salt and pepper. Sauté the onions for several minutes, until translucent. Add the shrimp, oregano, cumin, and a little more salt. Cook for about 4 minutes, or until the shrimp are almost cooked through. Add in the slices of roasted red peppers and the roasted corn. Stir until everything is evenly combined and hot.
2Warm the tortillas. Place the shrimp mixture in the tortillas. Top with the remaining diced onion, fresh cilantro, chunks of feta, and some sliced serrano pepper (adjust the amount according to how spicy you like your food). Serve the tacos with a wedge of lime. Squeeze the lime juice on top right before you eat the tacos.