Gorgonzola Steak Mac and Cheese

Name: Gorgonzola Steak Mac and Cheese
Ingredients: 2 medium white onions (sliced thin); 2 tbsp unsalted butter; 2 tbsp brown sugar; 3 tbsp balsamic vinegar; 1-1½ lb NY Strip steak (can sub ribeye or any cut with good marbling); 2 tsp onion powder; 1½ tsp coarse ground pepper; salt (to taste); 2 tbsp olive oil; 3 tbsp unsalted butter; 2 cloves garlic (smashed); 1 sprig rosemary (optional); 1 lb dried macaroni noodles (or cavatappi ); 3 tbsp all purpose flour; 3 cup whole milk (warmed on stovetop); 2 cup shredded white cheddar; 2 cups shredded monterey jack; 6 oz crumbled gorgonzola (plus more for garnish); ⅓ cup sliced green onion (for garnish)