Broccoli Soup With Cheddar Croutons

Name: Broccoli Soup With Cheddar Croutons
Ingredients: 2 heads broccoli with stems (about 1 lb. total); 2 tablespoons olive oil, plus more for serving; 2 large leeks, cut into ¼-in. slices (about 2 cups); 1 tablespoon kosher salt, divided; 4 cloves garlic, chopped; 2 Yukon Gold potatoes, peeled and cut into 1-in. cubes; 2 cups baby spinach; ¼ cup sour cream; 5 ounces sharp white Cheddar cheese, shredded (about 1¼ cups), divided; ¾ baguette (from an 8½-oz. loaf), cut into 12 (¼ in.) slices; Freshly ground black pepper, for serving