1. Heat oil in medium skillet over medium heat until hot and shimmering but not smoking, about 2 minutes; add onion and cook, stirring occasionally, until softened, about 4 minutes. Add garlic, spices, and 1/2 teaspoon salt; cook, stirring constantly, until fragrant, about 1 minute. Add ground beef and cook, breaking meat up with wooden spoon and scraping pan bottom to prevent scorching, until beef is no longer pink, about 5 minutes. Add tomato sauce, chicken broth, brown sugar, and vinegar; bring to simmer. Reduce heat to medium-low and simmer, uncovered, stirring frequently and breaking meat up so that no chunks remain, until liquid has reduced and thickened (mixture should not be completely dry), about 10 minutes. Adjust seasonings with salt and pepper.
2. Using wide, shallow spoon, divide filling evenly among taco shells; place two tacos on individual plates. Serve immediately, passing toppings separately.

Yield: 4 servings