Slice onion and place in a small bowl with vinegar and hot water while preparing rest of the ingredients. Heat 3 Tablespoons of broth over medium heat in a stainless steel skillet. While broth is heating, snap off the woody bottom of asparagus stems, then cut the spears into 2-inch lengths for quick even cooking. When broth begins to steam, add asparagus. Cover and cook for 5 minutes. The outside will be tender and the inside will be crisp. Thinner spears will take about 3 minutes. After about 10 minutes, remove onion from hot water and squeeze dry. Mix together roasted peppers with marinated onion, asparagus, vinegar, olive oil, salt, and pepper. Marinate for 4-5 minutes and serve warm.