Southwestern Stew on Soft Polenta

Name: Southwestern Stew on Soft Polenta
Ingredients: 1 tsp. ground cumin; 1/2 tsp. chili powder; 1/2 tsp. ground coriander; 3 cups vegetable broth; 2 1/2 cups peeled, cubed winter squash, such as butternut or delicata (3/4-inch pieces); 1 large all-purpose potato, peeled and cut into 3/4-inch cubes; 1 cup frozen corn; 1 bay leaf; 1/2 tsp. salt, or more to taste; 1 Tbs. tomato paste; 1/2 tsp. dried basil; 1 recipe Basic Polenta (soft version); 1 1/2 Tbs. all-purpose flour; 1 Tbs. plus 1 tsp. olive oil; 1 large onion, chopped; 2 medium cloves garlic, minced