1Place Cumin and Coriander seeds in small skillet. Toast on medium-high heat, just until fragrant, about 1 minute. Remove from skillet and cool completely. Coarsely crush using molcajete or mortar and pestle.
2Place cumin and coriander mixture, oil, juices, jalapeno, garlic, Worcestershire sauce and black pepper in blender container. Cover. Blend on high speed until smooth. Place steak in large baking dish. Add marinade, turning steak to coat. Place onion over top of steak. Cover with plastic wrap. Refrigerate at least 30 minutes
3Grill steak and onions over medium-high heat 6 to 8 minutes per side or until desired doneness. Let steak rest 5 minutes before slicing across the grain. Serve with grilled onions and warm tortillas. Garnish with chopped cilantro and fresh salsa, if desired.
Test Kitchen Tip: A molcajete is similar to a mortar and pestle. Traditional molcajetes are made from lava rock and come in different sizes for grinding spices and other ingredients used in traditional Mexican cuisine for recipes like salsas. Using a molcajete, versus a food processor or blender, helps to release all the essential oils in the ingredients and develops a richer flavor and more authentic texture.