Heat a large skillet over medium-high heat and add the olive oil. When the oil is hot, add the sliced bell pepper and onion. Sprinkle with taco seasoning.
Saute, stirring occasionally, until the peppers and onions have charred on some sides and are somewhat tender, about 5 minutes. Remove from the heat and set aside.
Reduce the heat to medium. Butter 4 slices of bread with about 1 tbsp butter per slice.
Place a slice of bread, buttered side down, in the skillet. Top with a handful of shredded Oaxaca Cheese, then top with about half of the sauteed peppers and onions. Top with another slice of bread, buttered side up.
Cook until the underside of the sandwich is golden brown and the cheese is mostly melted. Flip and cook for another couple of minutes, until each side is golden brown and crisp. Remove from the pan.
Repeat steps 4 and 5 with the remaining ingredients to make another grilled cheese sandwich.
Serve warm with chopped cilantro and salsa as a dipping sauce!