Char the poblanos over an open flame or under the broiler until blackened, turning with tongs as needed. Place in a medium bowl and cover with plastic wrap to allow the peppers to steam. Let sit for at least 5 minutes.
Meanwhile, thinly slice the onion. Heat the oil in a 10-inch cast-iron skillet over medium heat. Add the onions. Sprinkle with salt and oregano. Cook, stirring occasionally, until soft, 5 to 7 minutes.
Meanwhile, remove the peppers from the bowl. Cut each pepper in half lengthwise and lay them flat on the cutting board, skin side up. Using a knife, scrape the charred skin from the peppers. Flip the peppers and use the knife to scrape out the stems and seeds. Slice the peppers into 1/4-inch-thick slices.
Preheat the broiler. Add the sliced peppers and cream to the skillet with the onions and bring to a simmer. Once simmering, remove the skillet from the heat and stir in the cream cheese and Mexican crema until smooth. Stir in 1 1/2 cups of the Monterey Jack cheese until combined. Spread the mixture into an even layer and top with remaining 1/2 cup cheese. Place under the broiler and broil until bubbly and brown.
Remove the skillet from the oven. Garnish the dip with cilantro and sliced serrano. Serve with the tortilla chips for dipping.