How to Make It
Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion; sauté 5 minutes or until golden. Add chili powder, garlic powder, cumin, and oregano; cook 1 minute, stirring constantly. Add sirloin; cook 5 minutes, stirring to crumble. Add stock and salt; cook 4 minutes or until slightly thickened, stirring occasionally. Remove pan from heat. Divide beef mixture evenly among tostadas; top evenly with lettuce, tomato, and sour cream.
Prepare rice according to package directions. Combine rice, cilantro, and juice in a medium bowl. Serve with tostadas.