Sicilian Sweet & Sour Tilapia

Ingredients

• 2 red onions, thinly sliced
• 1 Tbs. + 2 Tbs. extra-virgin olive oil
• pinch red pepper flakes, more to taste
• ¼ tsp. + ¼ tsp. kosher salt
• 2 tsp. brown sugar
• ¼ cup red wine vinegar
• 4 6 oz. tilapia or other white fish fillets
• 2 Tbs. corn starch