Fill a non-stick skillet with 1” water, and place over medium high heat. Place sliced sweet potato in hot water and cook until fork tender about 2 to 4 minutes. Set aside.
To make red salsa: place tomatoes, onion, garlic, serrano, cilantro, stock and salt in blender. Blend until smooth. Heat up 2 teaspoon canola oil in non-stick sauce pan over medium high heat. Add salsa, cook for about 4 to 6 minutes or until tomato changes to a deeper red color.
Heat longaniza (or chorizo) over medium high heat in medium non-stick skillet. Cook for about 2 to 4 minutes or until meat is cooked; stirring frequently. Set aside. In same skillet, add red onions and sauté until softened, about 1 minute; stirring constantly. Add sweet potatoes and cumin, and stir until sweet potatoes are cooked through.
Warm tortillas for 10 to 15 seconds on each side on an ungreased pan
Assemble tacos by spooning sweet potato mixture onto the tortilla and topping with salsa roja and cotija cheese.