Preheat oven to 350 degrees F. In a large skillet cook onion in hot oil over medium heat about 5 minutes or until tender. Add tomatillos and garlic. Cook, covered, for 10 minutes, stirring occasionally. Uncover and cook 5 minutes more or until slightly thickened. Cool mixture to room temperature.
Meanwhile, combine brown sugar, chili powder, and 1/2 teaspoon of the salt. Rub evenly over steaks. Coat a 10-inch cast-iron skillet or other ovenproof skillet with cooking spray. Heat skillet over medium-high heat. Add steaks. Cook steaks for 5 minutes on each side or until nicely browned. Transfer skillet to oven; bake steaks for 20 minutes or until desired doneness (145 degrees F for medium-rare or 160 degrees F for medium), turning once.
Stir avocados, cilantro, red wine vinegar, jalapeno, and remaining 1/2 teaspoon salt into tomatillo mixture. Serve with steaks.