In a large skillet, heat over medium-high heat.
Add diced onions and sauté until translucent.
Stir in minced garlic and cook for an additional minute.
Add ground beef or chicken, breaking it apart with a spatula.
Season with cumin, chili powder, salt, and pepper.
Cook until the meat is browned and cooked through, about 7-10 minutes.
Warm the corn tortillas in a separate pan or microwave.
Assemble tacos by placing meat on tortillas.
Top with shredded lettuce, diced tomatoes, and cheese.
Serve with salsa and sour cream to taste.