Braised Short Ribs with Creamy Polenta

3 Tbs. olive oil 

6 lbs. meaty, bone-in, individual-cut short ribs 

4 tsp. kosher salt 

1 tsp. freshly ground pepper 

1 yellow onion, chopped 

2 carrots, diced 

6 cloves garlic, chopped  

1/3 cup all-purpose flour  

2 cups hearty red wine  

3 cups beef stock or broth  

2 Tbs. tomato paste 

1 Tbs. minced fresh rosemary   

1 bay leaf 

1 cup whole milk 

1 1/3 cups quick-cooking polenta 

1/2 cup freshly grated Parmigiano-Reggiano, plus more for serving