Melt butter in a skillet over medium heat.

Add onion, scallions, garlic, oregano, and cumin to the skillet.

Saute until onions are tender, about 5 minutes, stirring occasionally.

Transfer the onion mixture to a blender or food processor.

Add cilantro and chicken broth to the blender.

Puree until smooth.

Return the puree to the skillet.

Cook for 5 minutes, stirring until it comes to a boil.

Reduce heat to low.

Stir in the cornstarch mixture.

Continue to cook, stirring until the liquid returns to a boil.

Simmer for a few minutes until the sauce thickens.

Season with salt and pepper to taste.