Preheat your broiler to high.

Arrange the cored and halved tomatoes and serrano chiles on an aluminum foil-lined broiler pan, skin side up. Broil for 10 to 12 minutes, or until their skins char, shrivel, and they are soft.

Transfer the broiled tomatoes and chiles to a blender, then puree until smooth. Set aside.

Heat the olive oil in a medium saucepan over medium-high heat. Add the finely chopped yellow onion and minced garlic, and cook until soft, about 2 to 3 minutes.

Stir in the chopped fresh oregano leaves, ground cumin, and Worcestershire sauce.

Add the tomato puree to the saucepan and simmer for 10 minutes, or until the sauce has slightly thickened and the flavors have blended.

Season the salsa with salt to taste.

Serve warm or at room temperature with various chips and vegetables.