Preheat your oven to 375 degrees. Slice the poblano peppers in half lengthwise and scoop out the insides. Add the poblanos and onions to a baking sheet and toss with the oil and a pinch of salt and pepper. Arrange the peppers so that they are skin side up. Roast for 30 minutes.

Transfer the roasted peppers to a plastic bag or a bowl covered with plastic wrap. This allows peppers to steam and will loosen the tough outer skin. Once cool enough to handle, use your fingers to peel the skin away. Discard the papery skins.

Meanwhile, melt the butter in a medium skillet over medium high heat. Once melted, add the flour and whisk to form a thick paste. Cook for a few minutes until lightly browned. Then, slowly whisk in the chicken broth and cook for 5-10 minutes, until slightly thickened.

Add the creamy mixture to a blender or food processor along with the peeled peppers, roasted onions, sour cream, garlic, cumin and salt. Pulse until smooth. Taste and adjust with additional salt and pepper, if needed.