Cream of Broccoli Soup

Name: Cream of Broccoli Soup
Ingredients: 1 large stalk celery; 1 medium carrot; 1 medium yellow onion; 6 large cloves garlic; 1 medium Yukon gold potato (about 8 ounces); 1 1/4 pounds broccoli crowns (about 2 large or 4 small); 1 tablespoon olive oil; 1 tablespoon unsalted butter; 2 teaspoons kosher salt, divided; 1 teaspoon freshly ground black pepper, divided; Pinch red pepper flakes (optional); 6 cups low-sodium vegetable or chicken broth; 1 small bunch fresh chives (about 15), plus more for serving; 1/4 cup heavy cream, plus more for serving; Crispy roasted broccoli or toasted bread, for serving (optional)